[PHP]URL短縮用62進数ハッシュ

URL短縮サービスによく使われている手法をメモ
DBに登録された自動発番の数値を単純に0​-9,a-z,A-Zの62進数で変換します。

/**
 * 数値を0-9,a-z,A-Zの62進数に変換する
 */
function getEncodeToBase62($id, $base = 0)
{
    $sbox = array(  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61);
    $char = array( 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 97, 98, 99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122);
    $hash = array();
    $i = 0;
    do
    {
        $hash[$i] = chr( $char[$sbox[(int)( floor( $id / pow( 62, $i )) + $i ) % 62]] );
        $i = count($hash);
    } while(( $base > $i ) || ( pow( 62, $i ) <= $id ));

    return implode( "", $hash );
}