Let's have a nice look at the pros and cons of Flash:

Pros

Respect from people who aren't good at Flash
Good pay per job

Cons

Jobs can take forever to complete, lowering the amount of money you can make
To get the most out of it, you need to learn a scripting language anyways
Some people can't view Flash documents in a browser
Flash just pisses me off :P

Now let's take a look at the pros and cons of PHP:

Pros

Extremely versatile
Easy to get into, challenging to master
High demand for PHP coders
Can be done in any word processing application
You can work on your scripts from any computer, unlike with Flash
Works on all browsers that support HTML, since it converts the info to HTML before displaying it

Cons

Requires hosting with PHP support
Finger cramps :P

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Needless to say, PHP would be the more senseable route to choose if you're looking to make money