Check whether a string can be interpreted as a valid UTF-8 or if it has any non-ASCII UTF-8 bytes

isUTF8.php

<?php
/**
 * Check whether string $str can be interpreted as a valid UTF-8.
 * 
 * @param (string) $str A string to check for UTF-8
 * @param (bool)   $hasNonASCII_UTF8 If true, get non-ASCII bytes count in $str.
 *
 * @return (bool|int) if $str is not a valid UTF-8 string -> false
 *                    if $str is a valid UTF-8 string,
 *                    when $hasNonASCII_UTF8 == false -> true,
 *                    else $hasNonASCII_UTF8 == true  -> nr. of nonASCII bytes
 *
 * @author Dumitru Uzun
 */
function isUTF8($str, $hasNonASCII_UTF8 = false) {
    # ASCII                 # non-overlong 2-byte  # excluding overlongs      # straight 3-byte                 # excluding surrogates     # planes 1-3                  # planes 4-15             # plane 16
    // return preg_match('#^(?:[\x09\x0A\x0D\x20-\x7E]|[\xC2-\xDF][\x80-\xBF]|\xE0[\xA0-\xBF][\x80-\xBF]|[\xE1-\xEC\xEE\xEF][\x80-\xBF]{2}|\xED[\x80-\x9F][\x80-\xBF]|\xF0[\x90-\xBF][\x80-\xBF]{2}|[\xF1-\xF3][\x80-\xBF]{3}|\xF4[\x80-\x8F][\x80-\xBF]{2})*$#xs', $str);

    // Alternative
    // if ( !$hasNonASCII_UTF8 ) return preg_match('//u', $str);

    static $tests = array(
        "\xE0\xC0"                 => "\xC0\x80",
        "\xF0\xC0\xC0"             => "\xE0\x80\x80",
        "\xF8\xC0\xC0\xC0"         => "\xF0\x80\x80\x80",
        "\xFC\xC0\xC0\xC0\xC0"     => "\xF8\x80\x80\x80\x80",
        "\xFE\xC0\xC0\xC0\xC0\xC0" => "\xFC\x80\x80\x80\x80\x80",
    );

    for ($u = false, $i = 0, $l = strlen($str); $i < $l; ++$i) {
        if (($str[$i] & "\xC0") <= "\x80") {
            continue;
        }
        // ASCII or Async UTF-8 byte
        // Check sync UTF-8 bytes
        $s = substr($str, $i, 4);
        foreach ($tests as $t => $e) {
            ++$i;
            if (($t & $s) == $e) {
                $u += 1;
                continue 2;
            }
        }
        // Not an ASCII, nor sync UTF-8 bytes
        return false;
    }
    return $hasNonASCII_UTF8 ? $u : true;
}

Here is the JS implementation as utf8bytes