Redefine Class Methods or Class

It's called monkey patching. But, PHP doesn't have native support for it.

Though, as others have also pointed out, the runkit library is available for adding support to the language and is the successor to classkit. And, though it seemed to have been abandoned by its creator (having stated that it wasn't compatible with PHP 5.2 and later), the project does now appear to have a new home and maintainer.

I still can't say I'm a fan of its approach. Making modifications by evaluating strings of code has always seemed to me to be potentially hazardous and difficult to debug.

Still, runkit_method_redefine appears to be what you're looking for, and an example of its use can be found in /tests/runkit_method_redefine.phpt in the repository:

runkit_method_redefine('third_party_library', 'buggy_function', '',
    'return \'good result\''
);

runkit seems like a good solution but its not enabled by default and parts of it are still experimental. So I hacked together a small class which replaces function definitions in a class file. Example usage:

class Patch {

private $_code;

public function __construct($include_file = null) {
    if ( $include_file ) {
        $this->includeCode($include_file);
    }
}

public function setCode($code) {
    $this->_code = $code;
}

public function includeCode($path) {

    $fp = fopen($path,'r');
    $contents = fread($fp, filesize($path));
    $contents = str_replace('<?php','',$contents);
    $contents = str_replace('?>','',$contents);
    fclose($fp);        

    $this->setCode($contents);
}

function redefineFunction($new_function) {

    preg_match('/function (.+)\(/', $new_function, $aryMatches);
    $func_name = trim($aryMatches[1]);

    if ( preg_match('/((private|protected|public) function '.$func_name.'[\w\W\n]+?)(private|protected|public)/s', $this->_code, $aryMatches) ) {

        $search_code = $aryMatches[1];

        $new_code = str_replace($search_code, $new_function."\n\n", $this->_code);

        $this->setCode($new_code);

        return true;

    } else {

        return false;

    }

}

function getCode() {
    return $this->_code;
}
}

Then include the class to be modified and redefine its methods:

$objPatch = new Patch('path_to_class_file.php');
$objPatch->redefineFunction("
    protected function foo(\$arg1, \$arg2)
    {   
        return \$arg1+\$arg2;
    }");

Then eval the new code:

eval($objPatch->getCode());

A little crude but it works!

For the sake of completeness - monkey patching is available in PHP through runkit. For details, see runkit_method_redefine().