Is there a PHP syntax checker plugin for Notepad++?
Please don't answer "Use another editor instead"
Try NppExec plugin for Notepad++. Using it create a command to be something like this:
cmd.exe /K c:\your\path\to\php.exe -l "YOUR_FULL_FILE_NAME"
Instead of YOUR_FULL_FILE_NAME you should use appropriate Notepadd++ macro -- I think it is
$(FULL_CURRENT_PATH) but double check with NppExec manual (installs together with plugin).
P.S. But any IDE will be better for sure (I'm using PhpStorm). If IDE is too heavy for your PC then look for php-oriented editors, like Blumentals RapidPHP etc (it's lighter than full IDE but may have all really important features).