Let me see I get this right.
You send a html file with php code in it to your users.
If the user reads his/her mail, then you want a counter to go up by one.
If this is the case, then you cannot do this. php is parsed on the server and if you send the phpcode to the user then he/she will be able to read your code directly.
But what you can do is add this line to your html code <img src="www.yourwebserver.xx/countmein.php"> and let the script countmein.php increase the counter.