For the purposes of development and optimization, it is useful to check the compiled DLL file. It serves for several purposes;
First, the developer sees the classes and interfaces exposed from the compiled assembly.
Second, decompilation reveals how the compiler treats some PHP constructs and declarations, so it can be used from C# or other .NET languages. It also allows for detecting errors in or improving the compiler.
Third, performance critical pieces of code should be reviewed and, if possible, type information or a better decomposition should be added to the source code to improve the type analysis and thus the compilation result. See performance for more information.
The community project try.peachpie.io allows for an immediate look into the .NET byte code generated from PHP code. This is useful for the decompilation of small PHP snippets. The site internally (and in-memory) compiles the given PHP code and shows the byte code live, with an option to decompile to C# code.
A handy tool for decompiling entire assemblies. In case there is a large PHP project or a project consisting of more script files in general, ILSpy provides all the functionality needed for both checking the assembly and performance fine-tuning.
The tool gets error messages in case the compiled assembly is not valid (e.g. the compiler missbehavior) and gets nice view of the compiled assembly structure.
disasm is a basic tool getting all the metadata and IL stream of a compiled DLL file. Recommended for hardcore IL hunters.