CentOS exec():无法解决问题

In a php script, i am rotating sphinx indexes using exec command as below,

echo exec('/usr/bin/indexer --config /etc/sphinx/sphinx.conf --all --rotate');      

This exec called using a for loop, Most of the time, i got success message. But in some cases, i got warning message like below,

Severity: Warning

<p>Message:  exec(): Unable to fork [/usr/bin/indexer --config /etc/sphinx/sphinx.conf --all --rotate]</p>
<p>Filename: controllers/runscript.php</p>
<p>Line Number: 306</p>

Please suggest on this issue.

Regards, Raja MM