PHP exec() works in command line but not w

Question

I have the following line in a php file called batchrender.php:

exec('/Applications/Adobe\ After\ Effects\ CC\ 2014/aerender -project /Users/abc/Sites/AETEST1.aep -comp "Komp1" -output /Users/abc/Sites/xyz.mov');

When I call this file via web browser it should render a movie with the aerender unix command.

The output is

PROGRESS: ...After Effects successfully launched...

but that's all and it stops executing without any error. No movie is rendered. (No counting frames are displayed)

When I execute the exact same php file from the command line interface (Terminal) like:

 $ php batchrender.php

All works perfect and it starts to render the movie.

in the httpd file the user for apache2 is defined as "abc" and group is "staff" the terminal is also started with user "abc"