php - Why doesn't "exec" perform a command until I print its output? -

i onubuntu 14.04, php 5.5.9. there, have simple file a.php:

<?php  $a = exec('clear'); print($a);

if run script get:

$ php a.php  <screen cleared> # nothing displayed in screen                  # "clear" performed

however, if comment print($a) have file this:

<?php  $a = exec('clear');

then nothing happens:

$ php a.php  $               # nothing happened, see previous line above

to understanding, call variable $a makes clear performed. doesn't make sense, since exec() docs says:

exec — execute external program

is there reason why clear doesn't performed until print() called?

note related question clear cmd-shell php asking here why happens.

actually, clear gets executed when calling exec. don't see action on screen until output it.

EEE