<?php

//sometime string return this value
$string = 'mydomain.com/wp-content/uploads/2015/01/name-file-300x215.jpg';

//and sometime string return like this (original size)
// $string = 'mydomain.com/wp-content/uploads/2015/01/name-file.jpg';

//or sometime string return like this (external image link)
// $string = 'otherdomain.com/wp-content/uploads/2015/01/name-file.jpg';

//the point now, i want this newstring return like this
// $string = 'mydomain.com/wp-content/uploads/2015/01/name-file-90x90.jpg';

if(strpos($tring, 'mydomain.com/wp-content/uploads') !== false ){ //check if this string is internal image link

// SO WHAT TO DO BELOW?
$newstring = ''; // i want this $newstring return this value -> mydomain.com/wp-content/uploads/2015/01/name-file-90x90.jpg

//so i can $newstring (new image size 90x90) if string return internal image link
$return $newstring;
} else {
return $string;
}
?>