if you are using exec function, and have problems with a output > 1000 lines
you should use  

<?php
                stream_set_blocking($stream, true);
                while($line = fgets($stream)) {
                        flush();
                        echo $line."<br />";
                }
?>

except

<?php
                stream_set_blocking($stream, true);
               echo stream_get_contents($stream);
?>