Unicode Codepoint to UTF-8

unicode-encode-utf8.php

<?php
/**
 * @see Unicode 6.0.0 Ch2 General Structure, rfc3629
 * @param int|string $codepoint e.g. 0xC9 / "U+00C9"
 * @return string
 */
function unicodeCodePointToUTF8($codepoint)
{
    is_string($codepoint) && sscanf($codepoint, 'U+%x', $codepoint);
    if ($codepoint < 0) {
        throw new InvalidArgumentException('Lower than 0x00.');
    }
    if ($codepoint > 0x10FFFD) {
        throw new InvalidArgumentException('Larger than 0x10FFFD.');
    }
    if (0xD800 <= $codepoint && $codepoint <= 0xDFFF) {
        throw new InvalidArgumentException(sprintf('High and low surrogate halves are invalid unicode codepoints (U+D800 through U+DFFF, is U+%04X).', $codepoint));
    }
    if ($codepoint <= 0x7F) {
        return chr($codepoint);
    }
    if ($codepoint <= 0x7FF) {
        return chr(0xC0 | $codepoint >> 6 & 0x1F) . chr(0x80 | $codepoint & 0x3F);
    }
    if ($codepoint <= 0xFFFF) {
        return chr(0xE0 | $codepoint >> 12 & 0xF) . chr(0x80 | $codepoint >> 6 & 0x3F) . chr(0x80 | $codepoint & 0x3F);
    }
    return chr(0xF0 | $codepoint >> 18 & 0x7) . chr(0x80 | $codepoint >> 12 & 0x3F) . chr(0x80 | $codepoint >> 6 & 0x3F) . chr(0x80 | $codepoint & 0x3F);
}

Good snippet, also you can use json_encode function