In his latest post Derick Rethans talks about something that plagues every project, PHP or otherwise, after its grown to a large enough size: dead code. He's been asked why his Xdebug tool finds this code in places where people don't expect, so he figured he'd answer it once and for all.
The explanation for this is rather simple. Xdebug checks code coverage by adding hooks into certain opcodes. Opcodes are the building blocks of oparrays. PHP converts each element in your script - main body, method, function - to oparrays when it parses them. The PHP engine then executes those oparrays by running some code for each opcode. Opcodes are generated, but they are not optimised. Which means that it does not remove opcodes that can not be executed.
He gets down to the opcode level and shows some output from vld on how things are being executed (and what's not). Using a simple "foo" function example, he shows the execution flow and how the "branches" of executions work through the code. In his case, the "dead code" marker is coming from the line with a closing brace from an "if" statement. He points out that it entirely depends on the lines executed as to what is marked as "dead code".