Features of logical comparison in PHP

On April 4th, a question appeared on stackoverflow regarding the operation of comparison operators in PHP. Almost immediately he received a detailed response. Surely for many it is an interesting topic.

Question

PHP is famous for its type conversion. I spent a lot of time looking for the basics of the comparison logic in it.

For example: if $a > $b is true and $b > $c is true, does that mean that $a > $c also true?
')
Guided by the simplest logic, I can assume that this expression is also true, but I do not really trust PHP in this question. Can someone give me an example in which this statement is false?

I am also interested in the work of the “more” and “less” operators. Will the comparison result when turning the expression:

 # precondition: if ($a === $b) { throw new Exception( 'both are strictly equal, can not compare strictly for greater or smaller' ); } ($a > $b) !== ($b > $a)

For most combinations of types, the work of the comparison operators more / less is not documented.

Answer

The comparison operator in PHP at some points differs from the canonical definition:

The equality relation must be reflexive, symmetric, and transitive:

The == operator is not reflexive in PHP, i.e. $a == $a not always true:
```
 var_dump(NAN == NAN); // bool(false) 
```
Note: The fact that comparing with NAN is always false is not a feature of PHP. This behavior is defined in the IEEE 754 standard format for representing floating-point numbers ( explained on stackoverflow );
The operator == symmetric , that is, $a == $b and $b == $a always equal;

The operator == not transitive , that is, $a == $b and $b == $c does not mean that $a == $c :

 var_dump(true == "a"); // bool(true) var_dump("a" == 0); // bool(true) var_dump(true == 0); // bool(false)

The relation <= / >= should be non-reflexive, antisymmetric and transitive:

The <= operator in PHP is not reflexive , that is, the expression $a <= $a not always true (see the example for == );

The operator <= not antisymmetric , that is, the truth of the expressions $a <= $b and $b <= $a does not mean that $a == $b :

 var_dump(NAN <= "foo"); // bool(true) var_dump("foo" <= NAN); // bool(true) var_dump(NAN == "foo"); // bool(false)

The operator <= not transitive , that is, the truth of the expressions $a <= $b and $b <= $c does not mean that $a <= $c (the example is the same as for the == operator).

The <= operator <= not complete , that is, both $a <= $b and $b <= $a can be false:

 var_dump(new stdClass <= new DateTime); // bool(false) var_dump(new DateTime <= new stdClass); // bool(false)

The relation of strict inequality < / > should be anti-reflective, asymmetric and transitive:

The < operator in PHP is anti-reflective , that is, $a < $a always false. This is relevant since PHP 5.4. In previous versions, INF < INF is true;
The operator < not asymmetric , that is, the truth of the expression $a < $b does not mean that !($b < $a) is true (see the example for <= );
The operator < not transitive , that is, it does not follow from the truth of $a < $b and $b < $c that $a < $c also true:
```
 var_dump(-INF < 0); // bool(true) var_dump(0 < TRUE); // bool(true) var_dump(-INF < TRUE); // bool(false) 
```
In addition: The operator < not trichotomous , that is, the expressions $a < $b , $b < $a and $a == $b can be false (the example is the same as for <= );
In addition: The operator < can be looped around , i.e. there are cases when $a < $b , $b < $c and $c < $a are true:
```
 var_dump(INF < []); // bool(true) var_dump([] < new stdClass); // bool(true) var_dump(new stdClass < INF); // bool(true) 
```
Note: This example generates a warning of the "level of stdClass could not be converted to double" level notice.

You can find some amazing graphs in the PHP Sadness 52 article - Comparison Operators .

And in the end I want to note that two equalities in PHP are guaranteed (unlike almost everything else) because the interpreter leads them to the same form:

 ($a > $b) == ($b < $a) ($a >= $b) == ($b <= $a)

UPD: Second Answer

In PHP (relevant for version 5.5.0beta2) there is no strict comparison> == or <== with type checking, but there are several ways to compare types before comparing more / less:

Compare if ( gettype($a)===gettype($b) ) ... variable types if ( gettype($a)===gettype($b) ) ...
Explicitly cast the variables to the required type if ( (string)$a===(string)$b ) ...
Use manipulation with if ( ($a.'')===($b.'') ) ... types

Also note the following:

Floating-point numbers have limited accuracy;
The constants NAN and INF are of type float;
Comparing INF with INF is mathematically incorrect;
The numbers in the e-notation are of type float, even if they are small;
Integers above PHP_INT_MAX are automatically converted to floating point numbers;
Numbers with a floating point beyond the limits defined by the system contain INF;
Undeclared variables return NULL;
When assigning, integers starting with 0 are converted from octal to decimal;
When casting a string to an integer, it loses leading zeros.

Some specific comparisons

Unusual

  $a VS. $b $a>$b $a<$b $a<=$b $a>=$b $a==$b $a===$b float(NAN) float(-INF) false false false false false false float(NAN) float(0) false false false false false false float(NAN) float(1) false false false false false false float(NAN) float(INF) false false false false false false float(NAN) float(NAN) false false false false false false float(NAN) int(-1) false false false false false false float(NAN) int(0) false false false false false false float(NAN) int(1) false false false false false false

Translator's note: The answer above says that any comparison with NAN returns false, so there is nothing unusual here.

Equal, but not identical

  $a VS. $b $a>$b $a<$b $a<=$b $a>=$b $a==$b $a===$b NULL(NULL) array() false false true true true false NULL(NULL) bool(false) false false true true true false NULL(NULL) float(0) false false true true true false NULL(NULL) int(0) false false true true true false NULL(NULL) str('') false false true true true false array() bool(false) false false true true true false bool(false) float(0) false false true true true false bool(false) int(0) false false true true true false str('') bool(false) false false true true true false bool(false) str('0') false false true true true false float(-INF) bool(true) false false true true true false bool(true) float(1) false false true true true false float(INF) bool(true) false false true true true false float(NAN) bool(true) false false true true true false bool(true) int(-1) false false true true true false bool(true) int(1) false false true true true false bool(true) str("\0") false false true true true false bool(true) str('+') false false true true true false bool(true) str('-') false false true true true false bool(true) str('01') false false true true true false bool(true) str('1') false false true true true false bool(true) str('false') false false true true true false str('text') bool(true) false false true true true false str('true') bool(true) false false true true true false int(0) float(0) false false true true true false str("\0") float(0) false false true true true false str('') float(0) false false true true true false str('+') float(0) false false true true true false str('-') float(0) false false true true true false str('0') float(0) false false true true true false str('false') float(0) false false true true true false str('text') float(0) false false true true true false str('true') float(0) false false true true true false int(1) float(1) false false true true true false float(1) str('01') false false true true true false float(1) str('1') false false true true true false str("\0") int(0) false false true true true false str('') int(0) false false true true true false str('+') int(0) false false true true true false str('-') int(0) false false true true true false int(0) str('0') false false true true true false str('false') int(0) false false true true true false str('text') int(0) false false true true true false str('true') int(0) false false true true true false int(1) str('01') false false true true true false int(1) str('1') false false true true true false str('1') str('01') false false true true true false

And more and less at the same time?

  $a VS. $b $a>$b $a<$b $a<=$b $a>=$b $a==$b $a===$b float(NAN) str("\0") true true true true false false float(NAN) str('') true true true true false false float(NAN) str('+') true true true true false false float(NAN) str('-') true true true true false false float(NAN) str('0') true true true true false false float(NAN) str('01') true true true true false false float(NAN) str('1') true true true true false false float(NAN) str('false') true true true true false false float(NAN) str('text') true true true true false false float(NAN) str('true') true true true true false false

Identically

  $a VS. $b $a>$b $a<$b $a<=$b $a>=$b $a==$b $a===$b NULL(NULL) NULL(NULL) false false true true true true float(-INF) float(-INF) false false true true true true float(INF) float(INF) false false true true true true

More or less

  $a VS. $b $a>$b $a<$b $a<=$b $a>=$b $a==$b $a===$b NULL(NULL) bool(true) false true true false false false float(-INF) NULL(NULL) true false false true false false NULL(NULL) float(1) false true true false false false float(INF) NULL(NULL) true false false true false false float(NAN) NULL(NULL) true false false true false false NULL(NULL) int(-1) false true true false false false NULL(NULL) int(1) false true true false false false NULL(NULL) str("\0") false true true false false false NULL(NULL) str('+') false true true false false false NULL(NULL) str('-') false true true false false false NULL(NULL) str('0') false true true false false false NULL(NULL) str('01') false true true false false false NULL(NULL) str('1') false true true false false false NULL(NULL) str('false') false true true false false false NULL(NULL) str('text') false true true false false false NULL(NULL) str('true') false true true false false false array() bool(true) false true true false false false float(-INF) array() false true true false false false array() float(0) true false false true false false array() float(1) true false false true false false float(INF) array() false true true false false false float(NAN) array() false true true false false false array() int(-1) true false false true false false array() int(0) true false false true false false array() int(1) true false false true false false array() str("\0") true false false true false false str('') array() false true true false false false array() str('+') true false false true false false array() str('-') true false false true false false array() str('0') true false false true false false array() str('01') true false false true false false array() str('1') true false false true false false array() str('false') true false false true false false array() str('text') true false false true false false array() str('true') true false false true false false bool(true) bool(false) true false false true false false float(-INF) bool(false) true false false true false false float(1) bool(false) true false false true false false float(INF) bool(false) true false false true false false float(NAN) bool(false) true false false true false false bool(false) int(-1) false true true false false false int(1) bool(false) true false false true false false bool(false) str("\0") false true true false false false bool(false) str('+') false true true false false false bool(false) str('-') false true true false false false bool(false) str('01') false true true false false false str('1') bool(false) true false false true false false bool(false) str('false') false true true false false false str('text') bool(false) true false false true false false str('true') bool(false) true false false true false false bool(true) float(0) true false false true false false bool(true) int(0) true false false true false false str('') bool(true) false true true false false false bool(true) str('0') true false false true false false float(-INF) float(0) false true true false false false float(-INF) float(1) false true true false false false float(INF) float(-INF) true false false true false false float(-INF) int(-1) false true true false false false float(-INF) int(0) false true true false false false float(-INF) int(1) false true true false false false float(-INF) str("\0") false true true false false false float(-INF) str('') false true true false false false float(-INF) str('+') false true true false false false float(-INF) str('-') false true true false false false float(-INF) str('0') false true true false false false float(-INF) str('01') false true true false false false float(-INF) str('1') false true true false false false float(-INF) str('false') false true true false false false float(-INF) str('text') false true true false false false float(-INF) str('true') false true true false false false float(1) float(0) true false false true false false float(INF) float(0) true false false true false false float(0) int(-1) true false false true false false int(1) float(0) true false false true false false float(0) str('01') false true true false false false str('1') float(0) true false false true false false float(INF) float(1) true false false true false false float(1) int(-1) true false false true false false float(1) int(0) true false false true false false float(1) str("\0") true false false true false false str('') float(1) false true true false false false float(1) str('+') true false false true false false float(1) str('-') true false false true false false float(1) str('0') true false false true false false float(1) str('false') true false false true false false str('text') float(1) false true true false false false str('true') float(1) false true true false false false float(INF) int(-1) true false false true false false float(INF) int(0) true false false true false false float(INF) int(1) true false false true false false float(INF) str("\0") true false false true false false float(INF) str('') true false false true false false float(INF) str('+') true false false true false false float(INF) str('-') true false false true false false float(INF) str('0') true false false true false false float(INF) str('01') true false false true false false float(INF) str('1') true false false true false false float(INF) str('false') true false false true false false float(INF) str('text') true false false true false false float(INF) str('true') true false false true false false int(0) int(-1) true false false true false false int(1) int(-1) true false false true false false str("\0") int(-1) true false false true false false str('') int(-1) true false false true false false str('+') int(-1) true false false true false false str('-') int(-1) true false false true false false str('0') int(-1) true false false true false false int(-1) str('01') false true true false false false str('1') int(-1) true false false true false false str('false') int(-1) true false false true false false str('text') int(-1) true false false true false false str('true') int(-1) true false false true false false int(1) int(0) true false false true false false int(0) str('01') false true true false false false str('1') int(0) true false false true false false int(1) str("\0") true false false true false false str('') int(1) false true true false false false int(1) str('+') true false false true false false int(1) str('-') true false false true false false int(1) str('0') true false false true false false int(1) str('false') true false false true false false str('text') int(1) false true true false false false str('true') int(1) false true true false false false str('') str("\0") false true true false false false str('+') str("\0") true false false true false false str('-') str("\0") true false false true false false str("\0") str('0') false true true false false false str("\0") str('01') false true true false false false str('1') str("\0") true false false true false false str('false') str("\0") true false false true false false str('text') str("\0") true false false true false false str('true') str("\0") true false false true false false str('') str('+') false true true false false false str('') str('-') false true true false false false str('') str('0') false true true false false false str('') str('01') false true true false false false str('') str('1') false true true false false false str('') str('false') false true true false false false str('') str('text') false true true false false false str('') str('true') false true true false false false str('-') str('+') true false false true false false str('+') str('0') false true true false false false str('+') str('01') false true true false false false str('1') str('+') true false false true false false str('false') str('+') true false false true false false str('text') str('+') true false false true false false str('true') str('+') true false false true false false str('-') str('0') false true true false false false str('-') str('01') false true true false false false str('1') str('-') true false false true false false str('false') str('-') true false false true false false str('text') str('-') true false false true false false str('true') str('-') true false false true false false str('0') str('01') false true true false false false str('1') str('0') true false false true false false str('false') str('0') true false false true false false str('text') str('0') true false false true false false str('true') str('0') true false false true false false str('false') str('01') true false false true false false str('text') str('01') true false false true false false str('true') str('01') true false false true false false str('1') str('false') false true true false false false str('text') str('1') true false false true false false str('true') str('1') true false false true false false str('text') str('false') true false false true false false str('true') str('false') true false false true false false str('true') str('text') true false false true false false

Examples with $ a> $ b> $ c in which $ a is not more than $ c

 A<C : float(NAN) > str('a') > str('') A<C : float(NAN) > str('a') > str('1') A<C : float(NAN) > str('a') > str('A') A<C : float(NAN) > str('a') > str('0') A<C : float(NAN) > str('1') > str('') A<C : float(NAN) > str('1') > str('0') A<C : float(NAN) > str('A') > str('') A<C : float(NAN) > str('A') > str('1') A<C : float(NAN) > str('A') > str('0') A<C : float(NAN) > str('0') > str('') A==C : str('') > float(NAN) > NULL(NULL) A===C : str('') > float(NAN) > str('') A<C : str('') > float(NAN) > str('a') A<C : str('') > float(NAN) > str('1') A==C : str('') > float(NAN) > bool(false) A<C : str('') > float(NAN) > str('A') A<C : str('') > float(NAN) > str('0') A==C : str('') > float(-INF) > NULL(NULL) A==C : str('') > float(-INF) > bool(false) A==C : str('') > int(-1) > NULL(NULL) A==C : str('') > int(-1) > bool(false) A==C : str('') > float(-1) > NULL(NULL) A==C : str('') > float(-1) > bool(false) A==C : array() > float(NAN) > NULL(NULL) A==C : array() > float(NAN) > bool(false) A==C : array() > float(INF) > NULL(NULL) A==C : array() > float(INF) > bool(false) A==C : array() > float(-INF) > NULL(NULL) A==C : array() > float(-INF) > bool(false) A==C : array() > str('a') > NULL(NULL) A==C : array() > str('a') > bool(false) A==C : array() > int(1) > NULL(NULL) A==C : array() > int(1) > bool(false) A==C : array() > float(1) > NULL(NULL) A==C : array() > float(1) > bool(false) A==C : array() > str('1') > NULL(NULL) A==C : array() > str('1') > bool(false) A==C : array() > str('A') > NULL(NULL) A==C : array() > str('A') > bool(false) A==C : array() > str('0') > NULL(NULL) A==C : array() > int(-1) > NULL(NULL) A==C : array() > int(-1) > bool(false) A==C : array() > float(-1) > NULL(NULL) A==C : array() > float(-1) > bool(false) A===C : str('a') > float(NAN) > str('a') A<C : str('a') > str('') > float(NAN) A<C : str('a') > str('1') > float(NAN) A==C : str('a') > str('1') > int(0) A==C : str('a') > str('1') > float(0) A<C : str('a') > str('A') > float(NAN) A<C : str('a') > str('0') > float(NAN) A==C : bool(true) > str('') > float(NAN) A==C : bool(true) > str('') > float(-INF) A==C : bool(true) > str('') > int(-1) A==C : bool(true) > str('') > float(-1) A==C : bool(true) > array() > float(NAN) A==C : bool(true) > array() > float(INF) A==C : bool(true) > array() > float(-INF) A==C : bool(true) > array() > str('a') A==C : bool(true) > array() > int(1) A==C : bool(true) > array() > float(1) A==C : bool(true) > array() > str('1') A==C : bool(true) > array() > str('A') A==C : bool(true) > array() > int(-1) A==C : bool(true) > array() > float(-1) A==C : bool(true) > int(0) > float(-INF) A==C : bool(true) > int(0) > int(-1) A==C : bool(true) > int(0) > float(-1) A==C : bool(true) > str('0') > float(NAN) A==C : bool(true) > str('0') > float(-INF) A==C : bool(true) > str('0') > int(-1) A==C : bool(true) > str('0') > float(-1) A==C : bool(true) > float(0) > float(-INF) A==C : bool(true) > float(0) > int(-1) A==C : bool(true) > float(0) > float(-1) A==C : int(1) > str('a') > str('1') A==C : int(1) > str('A') > str('1') A==C : float(1) > str('a') > str('1') A==C : float(1) > str('A') > str('1') A<C : str('1') > float(NAN) > str('a') A===C : str('1') > float(NAN) > str('1') A<C : str('1') > float(NAN) > str('A') A<C : str('1') > str('') > float(NAN) A<C : str('1') > str('0') > float(NAN) A<C : str('A') > float(NAN) > str('a') A===C : str('A') > float(NAN) > str('A') A<C : str('A') > str('') > float(NAN) A<C : str('A') > str('1') > float(NAN) A==C : str('A') > str('1') > int(0) A==C : str('A') > str('1') > float(0) A<C : str('A') > str('0') > float(NAN) A==C : int(0) > float(-INF) > NULL(NULL) A==C : int(0) > float(-INF) > bool(false) A==C : int(0) > int(-1) > NULL(NULL) A==C : int(0) > int(-1) > bool(false) A==C : int(0) > float(-1) > NULL(NULL) A==C : int(0) > float(-1) > bool(false) A<C : str('0') > float(NAN) > str('a') A<C : str('0') > float(NAN) > str('1') A==C : str('0') > float(NAN) > bool(false) A<C : str('0') > float(NAN) > str('A') A===C : str('0') > float(NAN) > str('0') A==C : str('0') > float(-INF) > bool(false) A<C : str('0') > str('') > float(NAN) A==C : str('0') > int(-1) > bool(false) A==C : str('0') > float(-1) > bool(false) A==C : float(0) > float(-INF) > NULL(NULL) A==C : float(0) > float(-INF) > bool(false) A==C : float(0) > int(-1) > NULL(NULL) A==C : float(0) > int(-1) > bool(false) A==C : float(0) > float(-1) > NULL(NULL) A==C : float(0) > float(-1) > bool(false)

Funny string comparison: 'Queen' > 'King' > 'Jack' > 'Ace'

See also the table of type comparison in PHP in which comparisons are indicated:

isset() and is_null()
if () and empty ()
Differences in == and ===

The code that generated the lists on git hub
Differences in PHP versions: http://3v4l.org/MAfDu

Source: https://habr.com/ru/post/176117/

All Articles

Features of logical comparison in PHP

Question

Answer

UPD: Second Answer

Some specific comparisons

More articles: