Can't concatenate x codes (\x) - PHP

Question

I'm looking to use unpack().

This works:

$srbytes = "\x80\x3e\x00\x00";

$array1 = unpack("v",$srbytes); 

This does not:

$num1 = "80"

$srbytes = "\x".$num1."\x3e\x00\x00";

$array1 = unpack("v",$srbytes);

or

$srbytes = "\x"."80\x3e\x00\x00"; 
$array1 = unpack("v",$srbytes);

Printing this with echo shows ASCII chars with the first full string but, the concatenated ones shows text until it passes where it was concatenated.

Comparing a full string against a concatenated ones shows false, even though they should be the same?

what is actually happening when I'm trying to concatenate

Answer 1

The character expansion won't work, because at the point that you do "\\x" . "80" "\\x" . "80" PHP already has two string literals . It can't be expected to figure that meant anything else but this.

Instead of trying to concatenate a hexadecimal value for expansion, just concatenate the actual character, by converting the hexadecimal value to a base10 integer, and passing it to chr() , which converts it to an actual byte.

$str = "";
$num1 = "80";
$str .= chr(base_convert($num1, 16, 10));
var_dump($str);

Gives you

string(1) "�"

When you actually look at the value of $srbytes in your example where you define it as a string literal "\\x80\\x3e\\x00\\x00" , what you get is var_dump("\\x80\\x3e\\x00\\x00") giving you string(4) " >" , because PHP double quoted strings offer additional character expansion such as expanding on escaped hexadecimal values into bytes. However, var_dump("\\x"."80\\x3e\\x00\\x00") just gives you string(7) "\\x80>" , which is because the value "\\x" by itself is just a literal "\\x" as a string. So they aren't the same values, no.

Answer 2

If you want the 'literal' string use single quotes. Your issue is with escaped character sequences inside double quotes being evaluated. Example:

$srbytes = '\x'.'80\x3e\x00\x00'; 
echo $srbytes;
// \x80\x3e\x00\x00
var_dump($srbytes);
// string(16) "\x80\x3e\x00\x00" 

$srbytes = "\x"."80\x3e\x00\x00"; 
echo $srbytes;
// \x80>
var_dump($srbytes);
//string(7) "\x80>"

http://php.net/manual/en/language.types.string.php