A proper unicode string padder;

<?php
mb_internal_encoding('utf-8'); // @important

function str_pad_unicode($str, $pad_len, $pad_str = ' ', $dir = STR_PAD_RIGHT) {
    $str_len = mb_strlen($str);
    $pad_str_len = mb_strlen($pad_str);
    if (!$str_len && ($dir == STR_PAD_RIGHT || $dir == STR_PAD_LEFT)) {
        $str_len = 1; // @debug
    }
    if (!$pad_len || !$pad_str_len || $pad_len <= $str_len) {
        return $str;
    }
    
    $result = null;
    $repeat = ceil($str_len - $pad_str_len + $pad_len);
    if ($dir == STR_PAD_RIGHT) {
        $result = $str . str_repeat($pad_str, $repeat);
        $result = mb_substr($result, 0, $pad_len);
    } else if ($dir == STR_PAD_LEFT) {
        $result = str_repeat($pad_str, $repeat) . $str;
        $result = mb_substr($result, -$pad_len);
    } else if ($dir == STR_PAD_BOTH) {
        $length = ($pad_len - $str_len) / 2;
        $repeat = ceil($length / $pad_str_len);
        $result = mb_substr(str_repeat($pad_str, $repeat), 0, floor($length)) 
                    . $str 
                       . mb_substr(str_repeat($pad_str, $repeat), 0, ceil($length));
    }
    
    return $result;
}
?>

Test;
<?php
// needs ie. "test.php" file encoded in "utf-8 without bom"
$s = '...';
for ($i = 3; $i <= 1000; $i++) {
    $s1 = str_pad($s, $i, 'AO', STR_PAD_BOTH); // can not inculde unicode char!!!
    $s2 = str_pad_unicode($s, $i, 'ÄÖ', STR_PAD_BOTH);
    $sl1 = strlen($s1);
    $sl2 = mb_strlen($s2);
    echo  "len $sl1: $s1 \n";
    echo  "len $sl2: $s2 \n";
    echo  "\n";
    if ($sl1 != $sl2) die("Fail!");
}
?>

Output;
len 3: ... 
len 3: ... 

len 4: ...A 
len 4: ...Ä 

len 5: A...A 
len 5: Ä...Ä 

len 6: A...AO 
len 6: Ä...ÄÖ 
...