Source

File: wp-includes/formatting.php

function seems_utf8( $str ) {
	mbstring_binary_safe_encoding();
	$length = strlen( $str );
	reset_mbstring_encoding();
	for ( $i = 0; $i < $length; $i++ ) {
		$c = ord( $str[ $i ] );
		if ( $c < 0x80 ) {
			$n = 0; // 0bbbbbbb
		} elseif ( ( $c & 0xE0 ) == 0xC0 ) {
			$n = 1; // 110bbbbb
		} elseif ( ( $c & 0xF0 ) == 0xE0 ) {
			$n = 2; // 1110bbbb
		} elseif ( ( $c & 0xF8 ) == 0xF0 ) {
			$n = 3; // 11110bbb
		} elseif ( ( $c & 0xFC ) == 0xF8 ) {
			$n = 4; // 111110bb
		} elseif ( ( $c & 0xFE ) == 0xFC ) {
			$n = 5; // 1111110b
		} else {
			return false; // Does not match any model.
		}
		for ( $j = 0; $j < $n; $j++ ) { // n bytes matching 10bbbbbb follow ?
			if ( ( ++$i == $length ) || ( ( ord( $str[ $i ] ) & 0xC0 ) != 0x80 ) ) {
				return false;
			}
		}
	}
	return true;
}