用php在linux上执行sh脚本

I'm stuck with the following problem: I try to execute a sh script using php.

My attempts: I can run other commands. As an example I can run the 'cat' programm to display the script I'd like to execute: <?php echo exec("cat /srv/web/scripts/start_server.sh"); ?> The following snippets don't work:

<?php echo `/srv/web/scripts/start_server.sh`; ?>
<?php echo `sh /srv/web/scripts/start_server.sh`; ?>
<?php exec("sh /srv/web/scripts/start_server.sh"); ?>
<?php exec("/srv/web/scripts/start_server.sh"); ?>
The same with: system(), popen(), proc_open(), passthru(), shell_exec()

I tried various file permissions on the script. Even rwxrwxrwx and rwsrwxrwx. PHP continues execution after the execution as if it worked. No PHP error is generated. When I start the script on the console (like: sh /srv/web/scripts/start_server.sh) it works. I tested with a simple script, which works on the console but not when I try to execute it with php:

#!/bin/sh
echo hallo > /srv/web/scripts/file.txt

There is no safe-mode enabled. The exec function is nowhere disabled.

As an environment I use ipfire core 71 on an alix board. Apache as httpd and php5 as php interpreter.

I hope someone can help me.