Edit php.ini so that error reporting is on by default (log and/or output) and that error_level is set to report all errors, warnings etc, since you may have a parse error that prevents your script from executing at all.
However, you should also read the documentation for [man]exec[/man] since you may get output that you don't see. E.g. the last line may contain a whitespace.
You could of course also add extra output to determine how things really are proceeding, for example by echo 'before exec' before your line above, and echo 'after exec' after that line...