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Identical to the backtick operator ``, shell_exec($s) executes the external command $s and returns the output as a string. NULL is returned if an error occurred or the command produces no output.
exec($s [, &$arr [, &i]]) executes the external command $s, and stores the output lines and return status in the array $arr and the integer $i respectively. It returns the last line from the result of the command.

popen($s1, $s2) forks the external command $s1, and returns a file pointer that can be used by fgets(), fgetss(), and fwrite(). When $s2 is ‘r’, the returned file pointer equals STDOUT. When $s2 is ‘w’, the returned file pointer equals STDIN. pclose($r) closes a file pointer opened by popen().

RESETRUNFULL
<?php

echo `dir`;  // not single quotes!
echo shell_exec('dir');
shell_exec('dir',$arr);
$r = popen('dir','r');
echo fgets($r);

?>
proc_open($s1, $arr1, $arr2 [, $s2 [, $arr3 [, $arr4]]]) forks the external command $s1 and returns a file pointer. $arr1 is an indexed array where the keys can be 0(stdin), 1(stdout), or 2(stderr), and the value represents how PHP will pass that descriptor to the child process. The valid values are arrays with the first element being ‘pipe’ (the second element is ‘r’ or ‘w’) or ‘file’  (the second element is a filename).
$arr2 will be set to an index array of file pointers that correspond to PHP’s end of any pipes created.
$s2 is the absolute working directory of the command.
$arr3 is an array with the environment variables for the command that will be run, or NULL to use the same environment as the current PHP process.
$arr4 currently work for Windows only, and can include the keys ‘suppress_error’ (TRUE or FALSE) and ‘bypass_shell’ (TRUE or FALSE).
proc_close($r) closes the resource opened by proc_open(). 

<?php
$descriptorspec = array(
   0 => array("pipe", "r"),  // pipe that the child will read from
   1 => array("pipe", "w"),  // pipe that the child will write to
   2 => array("file", "/tmp/error-output.txt", "a") //file to write 
);

$cwd = '/tmp';
$env = array('some_option' => 'aeiou');

$process = proc_open('php', 
                                 $descriptorspec, $pipes, $cwd, $env);

if (is_resource($process)) {
    // $pipes now looks like this:
    // 0 => writeable handle connected to child stdin
    // 1 => readable handle connected to child stdout
    // Any error output will be appended to 
    //  /tmp/error-output.txt

    fwrite($pipes[0], '<?php print_r($_ENV); ?>');
    fclose($pipes[0]);

    echo stream_get_contents($pipes[1]);
    fclose($pipes[1]);

    // It is important that you close any pipes before calling
    // proc_close in order to avoid a deadlock
    $return_value = proc_close($process);

    echo "command returned $return_value\n";
}
?>
<!-- (courtesy of http://www.php.net/manual/en/function.proc-open.php)  --->