Calculator

We need a simple web-based calculator. It should cover 4 operations: add, subtract, multiply and divide. The calculator should be executed in the following manner:

http://localhost:8000/calculator.php?op=sum&x=3&y=4
http://localhost:8000/calculator.php?op=subtract&x=3&y=4
http://localhost:8000/calculator.php?op=multiply&x=3&y=4
http://localhost:8000/calculator.php?op=divide&x=3&y=4

The output should be the following:

Try to handle incorrect input data:

1. for the unrecognized operation your should show:
2. in case of any other problems you should show the following screens: