PHP exec bash script

I've written a bash script to take a screenshot and I want to launch it from a PHP page:

#!/bin/bash

screenshot="screnshot.png"
screencapture='/usr/sbin/screencapture -xC'

if [ `whoami` == 'root' ]; then
loginpid=`ps -ax | grep [l]oginwindow.app | awk '{print $1}'`
  launchctl bsexec $loginpid $screencapture $screenshot
else
  $screencapture $screenshot
fi

When I try to launch it from CLI it works as expected but when I try to launch it with PHP it doesn't work:

<?php
exec("bash /Users/giorgio/Desktop/src.sh");     
?>

What's the problem with this?

EDIT:

As you've suggested I've put the script into the PATH eviroment variable (I've edited .bash_profile). Now I can launch the command directly from CLI but the problem launching it from PHP stays the same.

I've tried with those commands but neither of them seems to work:

exec("bash /usr/local/bin/screenshot.sh");
exec("bash screenshot.sh"); 
exec("screenshot.sh");  

EDIT 2:

I've tried to run the following code to indagate on what is happening to the called script:

<?php
$array = array();
$integer;
exec("bash /usr/local/bin/screenshot.sh 2>&1",$array,$integer);     
echo "<pre>";
var_dump($integer);
echo "</pre>";
?>

It returns int(133); don't know what it means. P.s. I've also edited the shebang.

EDIT 3:

var_dump of $array returns this:

array(5) {
  [0]=>
  string(51) "dyld: Symbol not found: __cg_jpeg_resync_to_restart"
  [1]=>
  string(134) "  Referenced from: /System/Library/Frameworks/ApplicationServices.framework/Versions/A/Frameworks/ImageIO.framework/Versions/A/ImageIO"
  [2]=>
  string(59) "  Expected in: /Applications/MAMP/Library/lib/libJPEG.dylib"
  [3]=>
  string(0) ""
  [4]=>
  string(95) "/usr/local/bin/screenshot.sh: line 11: 44948 Trace/BPT trap          $screencapture $screenshot"
}