By default, monokai doesn't have any PHP-colour coding, it uses generic scopes such as comment, keyword, etc..
You can add colours for PHP-specific scopes by creating new rules in the theme-file. In a PHP file use Ctrl-Alt-Shift-P and read the current scope in the status-bar. Better yet, install the plug-in **ScopeHunter **so that you can copy and paste.
Although the order doesn't matter in a theme file I would add your new rules after all the keyword, comment, etc., rules, but before it gets on to nonsense like C/C++, diffs. But OF COURSE copy the theme file first, give it a new name (filename and the name towards the top of the file) and amend the code-number at the bottom (uuid - even though this is not essential..).
The scopes that you uncover will generally be quite long and specific. Create the rules firstly using the full scope-text to prove that it works; but you can then consider abbreviating the scope so that it catches more of your code. (The precise order and words used in your scope-rules are not hugely important, as long as it is sufficient to record a higher score than any other rule.)