从php运行bash脚本

Question

I have a raspberry pi connected to relay that switches lights. To switch the relay i use two bash scripts:

lighton.sh:

#!/bin/bash
echo 24 > /sys/class/gpio/export
gpio export 24 out

and lightoff.sh:

#!/bin/bash
echo 24 > /sys/class/gpio/unexport

When I execute scripts from console it works perfect, but... When I try to execute them from web interface, only "lighton.sh" script works. Actually I have more scripts that I want to run but the only one working script is "lighton.sh". This is the code of index.php:

<?php
if( isset( $_POST['lightOn'] ) )
{
    exec ('bash /absolute-path-to-script/lighton.sh');
}

if( isset( $_POST['lightOff'] ) )
{
    exec ('bash /absolute-path-to-script/lightoff.sh');
}
?> 

<form method="POST">
    <input name="lightOn" type="submit" value="Turn ON" />
    <input name="lightOff" type="submit" value="Turn OFF" />
</form>

I dont know why the only one script is working.

UPD: I have put buttons in different forms, but it still doesn't work. Sorry, i had a mistake in "codepasting" while i was writing this post(already corrected), but initial code is ok. So the problem is still actual.

UPD2: I have noticed that even if I put only one another button like:

<?php
if( isset( $_POST['lightOn'] ) )
{
    exec ('bash /absolute-path-to-script/lightoff.sh');
}
?> 

<form method="POST">
    <input name="lightOff" type="submit" value="Turn OFF" />
</form>

the button still doesn't work. Any other suggestions?