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You can use mb_strlen() to get the byte length using a encoding that only have byte-characters, without worring about multibyte or singlebyte strings. For example, as drake127 saids in a comment of mb_strlen, you can use '8bit' encoding:
<?php
$string = 'Cién cañones por banda';
echo mb_strlen($string, '8bit');
?>
You can have problems using strlen function since php have an option to overload strlen to actually call mb_strlen. See more info about it in http://php.net/manual/en/mbstring.overload.php
For trim the string by byte length without split in middle of a multibyte character you can use:
mb_strcut(string $str, int $start [, int $length [, string $encoding ]] )
Update for PHP 8.0 or great: Since PHP 8.0, Function Overloading was removed so you can always use strlen() to check the lenght in bytes.