- preg_replace_callback: documentation ( source)
- preg_replace: documentation ( source)
<?php
$text = 'KsKd2s4dAs =AhAd2s4dKs =AsAd2s4dKs 5s5d6s6d2c AsAd6s6d2c =AdQdKdJdTd =AhQhKhJhTh =AsQsKsJsTs';
echo preg_replace_callback('~=(\S+(?:\s+=\S+)*)~', function($m) {
return preg_replace('~\s+~', '', $m[1]);
}, $text);