utf8 encode a unicode code point

utf8encode.php

<?php
// utf8-encode a unicode code point
function utf8encode($cp){
    if($cp <= 0x7f){
        return pack("c", $cp);
    }
    else if($cp <= 0x7ff){
        return pack("cc",
            0b11000000 | (($cp >> 6) & 31),
            0b10000000 |  ($cp & 63)
        );
    }
    else if($cp<=0xffff){
        return pack("ccc",
            0b11100000 | (($cp >> 12) & 15),
            0b10000000 | (($cp >> 6) & 63),
            0b10000000 |  ($cp & 63)
        );
    }
    else if($cp<=0x1fffff){
        return pack("cccc",
            0b11110000 | (($cp >> 18) & 7),
            0b10000000 | (($cp >> 12) & 31),
            0b10000000 | (($cp >> 6) & 63),
            0b10000000 |  ($cp & 63)
        );
    }
}