truncate decimal

The truth is that I do not know what language is that (Math.truncate) but it is not php. I give you the answer in php, which is what you ask according to the label you have marked:

$valor //tu valor
$parte_entera = (int) $valor;
if($parte_entera == 5){
    $valor = $parte_entera;
}
else{
    $valor = round($valor);
}

Another way (the same in reality):

$valor = (floor($valor) == 5) ? 5 : round($valor);

You must use the round() php method. This method accepts three parameters: value, precision and mode.

With the third parameter, mode, you can achieve what you want like this:

if($numero == 5){
 $numero = round($numero, 2, PHP_ROUND_HALF_DOWN)
}

Here the documentation of the method: link