I've recently becoming interested in porting legacy PHP sites to JSPs. It seemed to me that one of the hardest parts of this problem was parsing the PHP code. Once a parse tree was created, the next step would be to emit equivalent JSP code.
I went looking for an ANTL4 grammar for PHP, but could only find an ANTLR3 grammar, so I went to work updating the ANTLR3 grammar to ANTLR4 and writing a very simple validation suite. The github project is here, and the resulting grammar is here.