file:///Users/jack/Desktop/Tester/login.php?login=yes
Others in have eluded to this, so I'll ask you straight out: Is this really your document root in MAMP? Sounds to me like you're by-passing MAMP altogher and your PHP code is not being parsed.
If you want to use MAMP's PHP install (and Apache and MySQL for that matter), you need to place your files in the document root or a sub folder thereof. Then point your browser to http://localhost:8888/.
You can change the document root MAMP uses in MAMP > Prefernces > Apache.