I agree with Abinadi that the tool by Mike kohn here is probably the best available still.
We did a successful conversion for a decent size project and wrote a blog about the process: Converting Classic ASP to PHP
While a standard lookup table with function could work it would be a LOT of work still to clean everything up. ASP to PHP is still probably one of the easier conversions but as mentioned will most likely end up with code that potentially is bad but in a different language.
Mike's tool handles fairly basic single page conversions and a good starting point but was outdated, missing a lot of functions and smarts when used on a bigger project. In saying that, it's still worth trying out even in the current state.
Here's a list of the main points we had to consider:
- Not all types have a compatible type, eg dates and booleans
- COM Objects can be used but may need heavy refactoring
- Variable case sensitivity (tools can help here a lot)
- Variable scoping (asp loves globals)
- HTML/JS Get and Post case sensitivity (harder to fix with tools)
- Object self references, eg PHP classes need $this->variable
- If you use lots of let/get/set be prepared for some heavier re-factoring
Of course the list above is just things to lookout for, if you were to create a tool you have to factor in a lot of the basics in parsing/tokenising asp code before even considering the above differences.
Good luck to anyone attempting this conversion project, having done it before we know the feeling.