File type association

 A little while ago I made a file and forgot to call it .php  (heroes) so it became a text file. Instead of deleting and starting again I renamed it heroes.php but it remained a text file. 

Oh well, tant pis. 

Except that now, every time I make a file called heroes.php it is a text file instead of a php  file. And it's a filename I use on most projects (or at least I used to)

Where is this association stored and how do I delete it?