How To Place PHP Inside A Standard Phrase

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Hi,

I'm venturing deeper into vBulletin and I'm wondering if this is possible.

I want to put a PHP script inside a standard phrase (its $vbphrase[registeremail] if you're wondering)

Now I know I can put this PHP script into a plugin (I think they're also called hooks??) and then call it that way.
But how would I call up the plugin like that? What is the code I'm suppose to use?

And also why can't I just paste in the PHP code straight into a phrase? Is this possible or does it conflict?

Any pointers are appreciated, I've been trying to learn all day how vBulletin works in alot greater detail.

Gilesy