xxxxxxxxxx
<?php
$string = "hello php";
$replace = str_replace(" ", "_", $string);
echo $replace; // hello_php
?>
xxxxxxxxxx
// Clean up multiple dashes or whitespaces
$string = preg_replace("/[\s-]+/", " ", $string);
// Convert whitespaces and underscore to dash
$string = preg_replace("/[\s_]/", "-", $string);