https://github.com/alecgorge/PHP-String-Class/issues/2

mb_str_replace_test.php

<?php

function mb_str_replace($search, $replace, $subject) {
		if(is_array($subject)) {
			$ret = array();
			foreach($subject as $key => $val) {
				$ret[$key] = mb_str_replace($search, $replace, $val);
			}
			return $ret;
		}
		foreach((array) $search as $key => $s) {
			if($s == '') {
				continue;
			}
			$r = !is_array($replace) ? $replace : (array_key_exists($key, $replace) ? $replace[$key] : '');
			$pos = mb_strpos($subject, $s);
			while($pos !== false) {
				$subject = mb_substr($subject, 0, $pos) . $r . mb_substr($subject, $pos + mb_strlen($s));
				$pos = mb_strpos($subject, $s, $pos + mb_strlen($r));
			}
		}
		return $subject;
	}

function mb_str_replace2($search, $replace, $subject){
        return preg_replace('@'.preg_quote($search).'@u',$replace,$subject);
}

$iteration = 100000;
$length = array();

$timestart = microtime(true);
for($i = 0; $i < $iteration; $i++) {

}
$length[] = microtime(true) - $timestart;

$timestart = microtime(true);
for($i = 0; $i < $iteration; $i++) {
	mb_str_replace("dog", "steak", "The quick dog dog dog dog dog dog dog dog dog dog brown fox jumps over the lazy dog.");
}
$length[] = microtime(true) - $timestart;

$timestart = microtime(true);
for($i = 0; $i < $iteration; $i++) {
	mb_str_replace2("dog", "steak", "The quick dog dog dog dog dog dog dog dog dog dog brown fox jumps over the lazy dog.");
}
$length[] = microtime(true) - $timestart;

var_dump($length);

output text.txt

pal-nat184-097-113:Downloads alecgorge$ php -f test.php 
array(3) {
  [0]=>
  float(0.0053048133850098)
  [1]=>
  float(4.1349000930786)
  [2]=>
  float(0.31121706962585)
}