<?php
/*
 * $s is assumed to be of the form
 *  STRING1:COUNT1/STRING2:COUNT2/..../STRINGN:COUNTN
 *
 * f returns a string that is COUNT1 copies of STRING1 followed by
 * COUNT2 copies of STRING2, etc.
 *
 * STRINGs may be empty; COUNTs must be >= 0
 */
function f($s)
{
    $result = "";
    $segs = explode("/", $s);
    
    $i = 0;
    while ($i < count($segs)) {
        $parts = explode(":", $segs[$i]);
        $result .= str_repeat($parts[0], $parts[1]);
        $i += 1;
        }
        
    return $result;
}

var_dump(f("a:3/b:10/xy:2"));
var_dump(f("10:0/1:10"));
var_dump(f(":1000000000000/xxxx:0")); # a trillion