Executing Python Script with PHP Variables

I am writing a simple application that uses information from a form, passes it through $_POST to a PHP script that executes a python script and outputs the results. The problem I am having is that my python script is not actually running with the arguments being passed in.

process3.php file:

<?php
     $start_word = $_POST['start'];
     $end_word = $_POST['end'];
     echo "Start word: ". $start_word . "<br />";
     echo "End word: ". $end_word . "<br />";
     echo "Results from wordgame.py...";
     echo "</br>";
     $output = passthru('python wordgame2.py $start_word $end_word');
     echo $output;
?>

Output:

Start word: dog
End word: cat
Results from wordgame.py...
Number of arguments: 1 arguments. Argument List: ['wordgame2.py']

At the top of my wordgame2.py, I have the following (for debugging purposes):

#!/usr/bin/env python
import sys
print 'Number of arguments:', len(sys.argv), 'arguments.'
print 'Argument List:', str(sys.argv)

Why isn't the number of arguments being passed = 3? (Yes, my form does send the data correctly.)

Any help is greatly appreciated!

Edit: I might add that it does run when I explicitly tell it the start and end word... something like this:

$output = passthru('python wordgame2.py cat dog');
echo $output