string must start with a letter

In this function $str must start with a letter. I used str_starts_with() function but I don't know how to set the parameters.

here are my codes:

<?PHP
function UsernameValidation(string $str = "_pouyaParsaei")
{
if (str_starts_with($str, '[a_Z]') === false) {
    return "false";
}
return $str;
}

echo UsernameValidation();