php中英文字符串转字母转大小写

<?php
    //转小写
    function lowercase($a){
        $b = str_split($a, 1);
        $r = '';
        foreach($b as $v){
            $v = ord($v);
            if($v >= 65 && $v<= 90){
                $v += 32;
            }
            $r .= chr($v);
        }
        return $r;
    }
    //转大写
    function capital($a){
        $b = str_split($a, 1);
        $r = '';
        foreach($b as $v){
            $v = ord($v);
            if($v >= 97 && $v<= 122){
                $v -= 32;
            }
            $r .= chr($v);
        }
        return $r;
    }
?>