public static function preSlash($str, $unify = false)
    {
        return self::_slash($str, '/', true, true, $unify);
    }
public static function preSlash($str, $unify = false)
    {
        return self::_slash($str, '/', true, true, $unify);
    }
public static function preSlash ( $ str , $ unify = false ) { return self :: _slash ( $ str , '/' , true , true , $ unify ) ; }
add slash to the front of the string.

@access public
@param  string  $str    the string to be slashed
@param  boolean $unify  set true to unify all slashes in $str
@return string          the slashed string
add slash to the front of the string .