函数是否一直被执行？

So I have a function in my wordpress custom theme witch I am using in all my forms. This function is in my wordpress functions.php and look like :

function mail_utf8($to, $subject = '(No subject)', $message = '', $from , $header = '') {
  $header_ = "From: $from" . "
" . 'MIME-Version: 1.0' . "
" . 'Content-type: text/plain; charset=UTF-8' . "
";
  mail($to, '=?UTF-8?B?'.base64_encode($subject).'?=', $message, $header_ . $header);
}

So the question is - Does this function is executing all the time, when some on refresh page? Or the function is executing only when I call it?

Because, I am getting from my host massages, that I have exceed the mail limit every hour. When I removed this function and hard coded in every form, everything is all right.

I tried :

if( !empty( $message ) ) {
 function mail_utf8($to, $subject = '(No subject)', $message = '', $from , $header = '') {
  $header_ = "From: $from" . "
" . 'MIME-Version: 1.0' . "
" . 'Content-type: text/plain; charset=UTF-8' . "
";
  mail($to, '=?UTF-8?B?'.base64_encode($subject).'?=', $message, $header_ . $header);
}
}

but it didnt work

If you don't have an opcode cache, this code is going to be parsed each time someone loads the page (loads/includes the file containing this code).

But it's only going to be executed when someone calls it.

Note: there is no point of specifying default arguments before $from because that field is mandatory.

Function executes only when something calls it. Calling function looks like some_fnc(arg1, arg2).

What you showed us is a function declaration. To actually call it one should write mail_utf8('some@email.com'). Look for it in your code and comment out if needed with \\ symbols.

What you did actually can cause fatal error now: when something will try to call mail_utf8() without $message set function will not be declared and script will exit with error:

PHP Fatal error:  Call to undefined function mail_utf8()