- str_replace: documentation ( source)
<?php
$string = '*_';
$specialCharsFrom = array('\\', '%', '&', '!', '@', '#', '$', '^', '!', '*', '_', '\'', "\n", ' ');
$specialCharsTo = array('\\\\', '\%', '&', '!', '@', '#', '$', '^', '!', '\*', '\_', '\'', '%', '%');
$searchStr = str_replace($specialCharsFrom, $specialCharsTo, $string);
echo $searchStr;