PHP implementation based on this SO answer: http://stackoverflow.com/a/742047/3393666

UrlShortener.php

<?php

class UrlShortener{

  private static $alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

  public static function encode($i){
    $alphabet = static::$alphabet;
    $base = strlen($alphabet);
    $result = "";
    do{
      $result = $alphabet[ $i % $base] . $result;
      //$i = intdiv($i, $base); //intdiv requires PHP >= 7
      $i = ($i - ($i % $base)) / $base;
    }while($i > 0);

    return $result;
  }

  public static function decode($s){
    $alphabet = static::$alphabet;
    $base = strlen($alphabet);
    $id = 0;
    $li = strlen($s)-1;
    $chars = str_split($s);
    foreach($chars as $c){
      $v = strpos($alphabet,$c);
      $id += $v * pow($base,$li--);
    }
    return $id;
  }

  public static function test(){
    for($i = 0; $i < 100000; $i++){
      $encoded = static::encode($i);
      $decoded = static::decode($encoded);
      if( $i != $decoded){
        echo "Error, $decoded != $i \n";
        exit(1);
      }else{
        echo "Good, $i ($encoded) == $decoded\n";
      }
    }
  }

}

UrlShortener::test();

//To test on command line:
//php UrlShortener.php

?>