Problem with rsvg-convert using php

Hello list, I know this is the development list, but is just a little 
question :)

Im traying to do a convertion, SVG to PDF, using rsvg-convert on PHP, Im 
doing an exec(), this is the command:
exec( "rsvg-convert --format=pdf 
/home/jfalvarez/php4/test/svgs/test_svg_tmp.svg --output 
/home/jfalvarez/php4/test/svgs/test_pdf.pdf" );

rsvg-convert permissions:
-rwxr-xr-x 1 root root 10016 2006-06-22 15:04 /usr/bin/rsvg-convert

I have the folder with all the permissions:
drwxrwxrwx 4 jfalvarez jfalvarez 4096 2007-01-23 19:42 svgs

The problem is, PHP can execute the command, but when you see the file 
size is 0 bytes:
-rw-r--r-- 1 www-data  www-data         0 2007-01-23 19:49 test_pdf.pdf

But if I execute the same command on my shell the pdf is right, I mean, 
like 9MB.

I dont think if some permissions problem, because the pdf file existe on 
the file system.

If somebody can help me :)

Thx for any help.

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