Dynamic generation of individual characters in an escape sequence

When working with character objects (control sequences, & -connections, escape sequences, mnemonics, etc.) in my case Escape-sequences of the type "\ xnn", it became necessary to form them dynamically. Such a task may arise, for example, when parsing a malicious code from an attacked site.



Actually the problem - the following code does not work:
')

$inx = '50'; $str = "\x".$inx; $str .= "\x51"; var_dump($str); //  \x50Q 

When parsing, I found out that this code works:

 $str = "\x50"; $str .= "\x51"; var_dump($str); //  PQ 

And the code in single quotes does not work at all, since the sequence is not interpreted at all:

 $str = '\x50'; $str .= '\x51'; var_dump($str); //  \x50\x51 

Formulation of the problem

Now the task can be formulated as follows: a proper interpretation of a string containing a dynamically generated sequence of type "\ xnn", whose characters are composed of fragments, is necessary.

In this case, two operations are performed — the concatenation and interpretation of character objects. And they are executed in strict sequence. First, the interpretation of character objects, then concatenation, but not vice versa.

Thus, in the first example, one of the encrypted characters of the sequence is divided into fragments.

"\ x51" is converted to the character "Q", and for the string "\ x" php searches for the next 2 characters, does not find them, and when combining the parts, outputs them to the string with 4 characters. Such a string is no longer a command for conversion for the php interpreter.

In the second example, php replaces the contents of double quotes with characters and "sticks together" them into a single string.

Decision

Since it was not possible to force php to first glue the characters together and then interpret them, we try to get the string as it is and replace the non-interpreted sequences with ready-made characters.

My searches for the built-in or finished function of converting a sequence of type "\ xnn" into characters did not lead to a distinct result. “They say that he was looking bad” (V. Tsoi). As a result, this problem was solved using the strtr () function. Here, the non-intrigued sequences '\ x50' are replaced with the convertible "\ x50". The only difference is in quotes.

Here is the finished function for character objects of type "\ xnn" and the call:

 function str2escape($string) { $sym_tbl = array( '\x00' => "\x00", '\x01' => "\x01", '\x02' => "\x02", '\x03' => "\x03", '\x04' => "\x04", '\x05' => "\x05", '\x06' => "\x06", '\x07' => "\x07", '\x08' => "\x08", '\x09' => "\x09", '\x0a' => "\x0a", '\x0b' => "\x0b", '\x0c' => "\x0c", '\x0d' => "\x0d", '\x0e' => "\x0e", '\x0f' => "\x0f", '\x11' => "\x11", '\x12' => "\x12", '\x13' => "\x13", '\x14' => "\x14", '\x15' => "\x15", '\x16' => "\x16", '\x17' => "\x17", '\x18' => "\x18", '\x19' => "\x19", '\x1a' => "\x1a", '\x1b' => "\x1b", '\x1c' => "\x1c", '\x1d' => "\x1d", '\x1e' => "\x1e", '\x1f' => "\x1f", '\x21' => "\x21", '\x22' => "\x22", '\x23' => "\x23", '\x24' => "\x24", '\x25' => "\x25", '\x26' => "\x26", '\x27' => "\x27", '\x28' => "\x28", '\x29' => "\x29", '\x2a' => "\x2a", '\x2b' => "\x2b", '\x2c' => "\x2c", '\x2d' => "\x2d", '\x2e' => "\x2e", '\x2f' => "\x2f", '\x31' => "\x31", '\x32' => "\x32", '\x33' => "\x33", '\x34' => "\x34", '\x35' => "\x35", '\x36' => "\x36", '\x37' => "\x37", '\x38' => "\x38", '\x39' => "\x39", '\x3a' => "\x3a", '\x3b' => "\x3b", '\x3c' => "\x3c", '\x3d' => "\x3d", '\x3e' => "\x3e", '\x3f' => "\x3f", '\x41' => "\x41", '\x42' => "\x42", '\x43' => "\x43", '\x44' => "\x44", '\x45' => "\x45", '\x46' => "\x46", '\x47' => "\x47", '\x48' => "\x48", '\x49' => "\x49", '\x4a' => "\x4a", '\x4b' => "\x4b", '\x4c' => "\x4c", '\x4d' => "\x4d", '\x4e' => "\x4e", '\x4f' => "\x4f", '\x51' => "\x51", '\x52' => "\x52", '\x53' => "\x53", '\x54' => "\x54", '\x55' => "\x55", '\x56' => "\x56", '\x57' => "\x57", '\x58' => "\x58", '\x59' => "\x59", '\x5a' => "\x5a", '\x5b' => "\x5b", '\x5c' => "\x5c", '\x5d' => "\x5d", '\x5e' => "\x5e", '\x5f' => "\x5f", '\x61' => "\x61", '\x62' => "\x62", '\x63' => "\x63", '\x64' => "\x64", '\x65' => "\x65", '\x66' => "\x66", '\x67' => "\x67", '\x68' => "\x68", '\x69' => "\x69", '\x6a' => "\x6a", '\x6b' => "\x6b", '\x6c' => "\x6c", '\x6d' => "\x6d", '\x6e' => "\x6e", '\x6f' => "\x6f", '\x71' => "\x71", '\x72' => "\x72", '\x73' => "\x73", '\x74' => "\x74", '\x75' => "\x75", '\x76' => "\x76", '\x77' => "\x77", '\x78' => "\x78", '\x79' => "\x79", '\x7a' => "\x7a", '\x7b' => "\x7b", '\x7c' => "\x7c", '\x7d' => "\x7d", '\x7e' => "\x7e", '\x7f' => "\x7f", '\x81' => "\x81", '\x82' => "\x82", '\x83' => "\x83", '\x84' => "\x84", '\x85' => "\x85", '\x86' => "\x86", '\x87' => "\x87", '\x88' => "\x88", '\x89' => "\x89", '\x8a' => "\x8a", '\x8b' => "\x8b", '\x8c' => "\x8c", '\x8d' => "\x8d", '\x8e' => "\x8e", '\x8f' => "\x8f", '\x91' => "\x91", '\x92' => "\x92", '\x93' => "\x93", '\x94' => "\x94", '\x95' => "\x95", '\x96' => "\x96", '\x97' => "\x97", '\x98' => "\x98", '\x99' => "\x99", '\x9a' => "\x9a", '\x9b' => "\x9b", '\x9c' => "\x9c", '\x9d' => "\x9d", '\x9e' => "\x9e", '\x9f' => "\x9f", '\xa1' => "\xa1", '\xa2' => "\xa2", '\xa3' => "\xa3", '\xa4' => "\xa4", '\xa5' => "\xa5", '\xa6' => "\xa6", '\xa7' => "\xa7", '\xa8' => "\xa8", '\xa9' => "\xa9", '\xaa' => "\xaa", '\xab' => "\xab", '\xac' => "\xac", '\xad' => "\xad", '\xae' => "\xae", '\xaf' => "\xaf", '\xb1' => "\xb1", '\xb2' => "\xb2", '\xb3' => "\xb3", '\xb4' => "\xb4", '\xb5' => "\xb5", '\xb6' => "\xb6", '\xb7' => "\xb7", '\xb8' => "\xb8", '\xb9' => "\xb9", '\xba' => "\xba", '\xbb' => "\xbb", '\xbc' => "\xbc", '\xbd' => "\xbd", '\xbe' => "\xbe", '\xbf' => "\xbf", '\xc1' => "\xc1", '\xc2' => "\xc2", '\xc3' => "\xc3", '\xc4' => "\xc4", '\xc5' => "\xc5", '\xc6' => "\xc6", '\xc7' => "\xc7", '\xc8' => "\xc8", '\xc9' => "\xc9", '\xca' => "\xca", '\xcb' => "\xcb", '\xcc' => "\xcc", '\xcd' => "\xcd", '\xce' => "\xce", '\xcf' => "\xcf", '\xd1' => "\xd1", '\xd2' => "\xd2", '\xd3' => "\xd3", '\xd4' => "\xd4", '\xd5' => "\xd5", '\xd6' => "\xd6", '\xd7' => "\xd7", '\xd8' => "\xd8", '\xd9' => "\xd9", '\xda' => "\xda", '\xdb' => "\xdb", '\xdc' => "\xdc", '\xdd' => "\xdd", '\xde' => "\xde", '\xdf' => "\xdf", '\xe1' => "\xe1", '\xe2' => "\xe2", '\xe3' => "\xe3", '\xe4' => "\xe4", '\xe5' => "\xe5", '\xe6' => "\xe6", '\xe7' => "\xe7", '\xe8' => "\xe8", '\xe9' => "\xe9", '\xea' => "\xea", '\xeb' => "\xeb", '\xec' => "\xec", '\xed' => "\xed", '\xee' => "\xee", '\xef' => "\xef", '\xf1' => "\xf1", '\xf2' => "\xf2", '\xf3' => "\xf3", '\xf4' => "\xf4", '\xf5' => "\xf5", '\xf6' => "\xf6", '\xf7' => "\xf7", '\xf8' => "\xf8", '\xf9' => "\xf9", '\xfa' => "\xfa", '\xfb' => "\xfb", '\xfc' => "\xfc", '\xfd' => "\xfd", '\xfe' => "\xfe", '\xff' => "\xff", ); return strtr($string, $sym_tbl); } // : $str = str2escape($str); 

Now everything is fine:

 $inx = '50'; $str = "\x".$inx; $str .= "\x51"; var_dump(str2escape($str)); //  PQ 

User Habra-Mikhail offered a variant without using his own function

 $inx = '50'; $str = chr(hexdec($inx)); $str .= "\x51"; var_dump($str); //  PQ 

The search for the cause and the solution took a certain amount of time, I think the path I have gone through will help everyone who comes across this task in the future to quickly understand.