This launches the command $cmd, redirects the command output to $outputfile, and writes the process id to $pidfile.

exec(sprintf("%s > %s 2>&1 & echo $! >> %s", $cmd, $outputfile, $pidfile));

This lets you easily monitor what the process is doing and if it’s still running.

function isRunning($pid){
    try{
        $result = shell_exec(sprintf("ps %d", $pid));
        if( count(preg_split("/\n/", $result)) > 2){
            return true;
        }
    }catch(Exception $e){}

    return false;
}

 Source

Manual:

exec

proc_open

$user_id = (is_user_logged_in() && 
!empty($current_user)) ? $current_user->ID : '';
		
if(empty($user_id)){	
  $current_user = wp_get_current_user();
  if($current_user instanceof WP_User){
    $user_id = $current_user->ID;
  }
}

/**
 * Checks if a particular user has a role. 
 * Returns true if a match was found.
 *
 * @param string $role Role name.
 * @param int $user_id (Optional) The ID of a user. Defaults to the current user.
 * @return bool
 */
function appthemes_check_user_role( $role, $user_id = null ) {

    if ( is_numeric( $user_id ) )
	$user = get_userdata( $user_id );
    else
        $user = wp_get_current_user();

    if ( empty( $user ) )
	return false;

    return in_array( $role, (array) $user->roles );
}

// example use for the current user
if ( appthemes_check_user_role( 'customer' )
    _e( "You've got access dude!", 'appthemes' );
else
    _e( "Sorry man, no luck.", 'appthemes' );

// example use for a specific user
$user_id = 23;

if ( appthemes_check_user_role( 'customer', $user_id )
    _e( "You've got access dude!", 'appthemes' );
else
    _e( "Sorry man, no luck.", 'appthemes' );