bruteforce.php

<?php
	
$str = 'fD3_';
$chars = array_merge(range('a', 'z'), range('A', 'Z'), range('0', '9'), ['_']);
$total = 0;
$brut = '';
$len = strlen($str);
/**
* @see: https://www.programmingalgorithms.com/algorithm/brute-force?lang=PHP
*/
function BruteForce($testChars, $startLength, $endLength, $testCallback) {
	for ($len = $startLength; $len <= $endLength; ++$len) {
		for ($i = 0; $i < $len; ++$i) $chars[$i] = $testChars[0];

		if ($testCallback($chars)) return implode("", $chars);

		for ($i1 = $len - 1; $i1 > -1; --$i1) {
			$i2 = 0;
			$testCharsLen = strlen($testChars);
			
			for ($i2 = strpos($testChars, $chars[$i1]) + 1; $i2 < $testCharsLen; ++$i2) {
				$chars[$i1] = $testChars[$i2];

				if ($testCallback($chars)) return implode("", $chars);

				for ($i3 = $i1 + 1; $i3 < $len; ++$i3) {
					if ($chars[$i3] != $testChars[$testCharsLen - 1]) {
						$i1 = $len;
						break 2;
					}
				}
			}
			if ($i2 == $testCharsLen) $chars[$i1] = $testChars[0];
		}
	}

	return false;
}
$brut = BruteForce(implode("", $chars), 1, $len, function ($testChars) use($str, &$total) {
	$total++;
	return strcmp(implode($testChars), $str) == 0;
});
print sprintf("Full string: %s => %d\r\n", $brut, $total);

$total = 0;
$brut = '';
$len = strlen($str);
for($i = 0; $i < $len; $i++){
	foreach($chars as $char){
		$total++;
		if(ord($char) == ord($str[$i])){
			$brut .= $char;
			continue 2;
		}
	}
}
print sprintf("By char: %s => %d\r\n", $brut, $total);