Warning: preg_replace_callback(): Requires argument 2 - php

Question

I just updated PHP on my server from php 5 to php 7 and I'm getting these warnings:
Warning: preg_replace_callback() [function.preg-replace-callback0]: Requires argument 2, 'chr(\1)', to be a valid callback
Warning: preg_replace_callback() [function.preg-replace-callback0]: Requires argument 2, 'chr(0x\1)', to be a valid callback
Warning: preg_replace_callback() [function.preg-replace-callback0]: Requires argument 2, 'chr(\1)', to be a valid callback
Warning: preg_replace_callback() [function.preg-replace-callback0]: Requires argument 2, 'chr(0x\1)', to be a valid callback
This is the PHP code:
private function _decode( $source )
{
$source = html_entity_decode($source, ENT_QUOTES, 'UTF-8');
$source = preg_replace_callback('/&#(\d+);/me',"chr(\\1)", $source);
$source = preg_replace_callback('/&#x([a-f0-9]+);/mei',"chr(0x\\1)", $source);
return $source;
}
The Warning is come from:
$source = preg_replace_callback('/&#x([a-f0-9]+);/mei',"chr(0x\\1)", $source);
How i can fix this?

Answer 1

The /e modifier (PREG_REPLACE_EVAL) is no longer supported, as noted in the PHP 7.0 migration guide. You need to use a callable function, not a string that will be evaluated as a function. In your case, replace your string function -- chr(0x\\1) -- with a Closure:
$source = preg_replace_callback(
'/&#x([a-f0-9]+);/mi',
fn($m) => chr(hexdec('0x'.$m[1])),
$source
);
The inline string replacement of \\1 to yield a valid PHP hexadecimal, like 0x21, no longer works that way in the callable: you need a hexdec call to accomplish the same.
See it in action on 3v4l.org.
If you do not yet have PHP 7.4 with short closures, you need to write that as:
$source = preg_replace_callback(
'/&#x([a-f0-9]+);/mi',
function ($m) { return chr(hexdec('0x'.$m[1])); }, // Now a Closure
$source
);