How to change this internal image link parameter using PHP? i want to resize

How resize this string.

Original file = mydomain.com/wp-content/uploads/2015/01/name-file.jpg

Return to $string now = mydomain.com/wp-content/uploads/2015/01/name-file-300×215.jpg

But, i want this size return to (-90×90) = mydomain.com/wp-content/uploads/2015/01/name-file-90×90.jpg

<?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 return this $newstring (new image size 90x90) if string is internal image link
    $return $newstring;
    } else {
//external image, no need change link parameter
    return $string;
    }
    ?>