PHP convert special characters to HTML entity

Question

I have a string ex:

$a = 'abc🔹abc';

The 'small blue diamond' is: bin2hex('🔹') => f09f94b9

Small blue diamond representation

So, I would like to convert the $a string into a string which represents the small blue diamond with the HTML-escape: 🔹

What would be the function what I should call to convert all unicode character into the HTML-escape representation?

More details on this case

In WordPress when I want to insert the $a variable into a table, $wpdb does it checks. Link to WPDB source code

When WordPress prepares the $data which should be inserted or updated, it runs the fields on the $wpdb->strip_invalid_text method and then it check if anything invalid found in the $data. It the text in the $a variable invalid with the following regexp:

                $regex = '/
                (
                    (?: [\x00-\x7F]                  # single-byte sequences   0xxxxxxx
                    |   [\xC2-\xDF][\x80-\xBF]       # double-byte sequences   110xxxxx 10xxxxxx
                    |   \xE0[\xA0-\xBF][\x80-\xBF]   # triple-byte sequences   1110xxxx 10xxxxxx * 2
                    |   [\xE1-\xEC][\x80-\xBF]{2}
                    |   \xED[\x80-\x9F][\x80-\xBF]
                    |   [\xEE-\xEF][\x80-\xBF]{2}';
            if ( 'utf8mb4' === $charset ) {
                $regex .= '
                    |    \xF0[\x90-\xBF][\x80-\xBF]{2} # four-byte sequences   11110xxx 10xxxxxx * 3
                    |    [\xF1-\xF3][\x80-\xBF]{3}
                    |    \xF4[\x80-\x8F][\x80-\xBF]{2}
                ';
            }
            $regex .= '){1,40}                          # ...one or more times
                )
                | .                                  # anything else
                /x';
            $value['value'] = preg_replace( $regex, '$1', $value['value'] );
            if ( false !== $length && mb_strlen( $value['value'], 'UTF-8' ) > $length ) {
                $value['value'] = mb_substr( $value['value'], 0, $length, 'UTF-8' );
            }

When the 'small blue diamond' represented with f09f94b9, this regexp marks the data invalid. When it is represented with 🔹. So what I need is to convert that unicode characters into a representation what is accepted by WordPress.

Answer 1

Here is what I came up with to convert all of the characters you can modify it further to only convert characters in the range you need.

$s = 'abc🔹def';
$a = preg_split('//u', $s, null, PREG_SPLIT_NO_EMPTY);

foreach($a as $c){
    echo '&#' . unpack('V', iconv('UTF-8', 'UCS-4LE', $c))[1] . ';';
}