Tests whether a string complies as UTF-8. This will be much faster than utf8_is_valid but will pass five and six octet UTF-8 sequences, which are not supported by Unicode and so cannot be displayed correctly in a browser. In other words it is not as strict as utf8_is_valid but it's faster. If you use it to validate user input, you place yourself at the risk that attackers will be able to inject 5 and 6 byte sequences (which may or may not be a significant risk, depending on what you are are doing)