// Get the charset to be used, the longer the charset the shortener the output will be
function charset($length = 36) {
	switch ($length) {
		case '36':
			$charset = '0123456789abcdefghijklmnopqrstuvwxyz';
			break;

		case '64':
			$charset = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
			break;
		
		default:
			$charset = '0123456789abcdefghijklmnopqrstuvwxyz';
			break;
	}
	
	return $charset;
}

/**
 * Convert an integer (1337) to a string (3jk)
 *
 */
function int2string( $num, $chars = null ) {
	if( $chars == null )
		$chars = $this->charset();
	$string = '';
	$len = strlen( $chars );
	while( $num >= $len ) {
		$mod = bcmod( $num, $len );
		$num = bcdiv( $num, $len );
		$string = $chars[ $mod ] . $string;
	}
	$string = $chars[ intval( $num ) ] . $string;

	return $string;
}

/**
 * Convert a string (3jk) to an integer (1337)
 *
 */
function string2int( $string, $chars = null ) {
	if( $chars == null )
		$chars = $this->charset();
	$integer = 0;
	$string = strrev( $string  );
	$baselen = strlen( $chars );
	$inputlen = strlen( $string );
	for ($i = 0; $i < $inputlen; $i++) {
		$index = strpos( $chars, $string[$i] );
		$integer = bcadd( $integer, bcmul( $index, bcpow( $baselen, $i ) ) );
	}
	return $integer;
}