How to remove appended PHP code to file using script

Question

I've got multiple files that look like this:

<?php
if($_SERVER["SCRIPT_NAME"] != "/index.php"){ header("HTTP/1.0 403 Forbidden");echo base64_decode("PCF..redacted.HRtbD4=");die(); }
?>
<?php
/**
 * actual PHP code starts here
*/
phpinfo();

I've got a script to clean these files which looks like this:

 sed -i -e '/if($_SERVER["SCRIPT_NAME"] != "/index.php")/d' -e '/header("HTTP\/1.0 403 Forbidden");echo base64_decode(/d'

However, with this solution, my resulting "clean" file looks like this:

<?php
?>
<?php
/**
 * actual PHP code starts here
*/
phpinfo();

So I'd like to remove the (now duplicate) PHP tags, I could just delete the line before and after my match but I want to make sure I'm deleting the duplicate tabs otherwise the scripts will stop working.

Thanks