Last I'd heard (been a while...), even the method you show for the subject wasn't browser-safe (I don't know if the Gecko family of browsers has gotten around this or not, and I've not tested).
Why not use your PHP skills to create a web form that does the whole shebang for you?
Otherwise, it might be possible with Javascript, as it's client side, but I'm not the one to ask about that, for sure....
[edit]but, y'know what ... I just tried this with Mozilla Browser/Mail ... and it worked!
<a href=mailto:me@mydomain?subject=test&body=This%20message%20is%20automated.">
Click
</a>
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