BB Code using PHP?

[Jibba Jabbas]

How do i go about using PHP when creating new BB Codes? Basically i want to create a new BB Code and i need to use the text between it in a PHP script...

So the process is like this:

[bbcode]test[bbcode]

Then a PHP script is run which has say $x = the user input.. in this case "test".

Then i after using that input to gather the relevant data i have it all in $a and want to print the contents of $a on the screen.

So it goes from [bbcode]test[/bbcode] to showing the contents of $a.

How do i go about doing this?

Thanks