<?php

function calculateRelativePath($a,$b) {
    if (empty($a) || empty($b))
        return;
    $flag = "";
    $split1 = explode("/",$a);
    $split2 = explode("/",$b);
    $diff1 = array_diff_assoc($split1, $split2);
    $count = count($diff1 );
    for($i=0; $i < $count-1; $i++) {
        $flag .= "../";
    } 
    $diff2 = array_diff_assoc($split2, $split1);
    return $flag. implode("/",$diff2);
}
$relative = calculateRelativePath("/a/b/c/d/e.php","/a/b/12/34/c.php");
print_r($relative);