html output done without result of exec_shell

Guys, I'm trying to output the result of a shell command within a page. Problem is, when it's a command with short execution time like echo shell_exec("ls -la"); it works fine, but the command I'm really trying to execute takes 2-3 seconds before having calculated the response and by then, the page is already done and the output of the command isn't included. I played around with the settings in php.ini, but none of them changed the problem. Maybe an Apache problem? What am I missing? Thanks in advance. Regards, Gunther E. Biernat