Execute shell commands in PHP

<?php // Source: http://www.binarytides.com/execute-shell-commands-php/

/**
    Method to execute a command in the terminal
    Uses :

    1. system
    2. passthru
    3. exec
    4. shell_exec

*/
function terminal($command)
{
    //system
    if(function_exists('system'))
    {
        ob_start();
        system($command , $return_var);
        $output = ob_get_contents();
        ob_end_clean();
    }
    //passthru
    else if(function_exists('passthru'))
    {
        ob_start();
        passthru($command , $return_var);
        $output = ob_get_contents();
        ob_end_clean();
    }

    //exec
    else if(function_exists('exec'))
    {
        exec($command , $output , $return_var);
        $output = implode("n" , $output);
    }

    //shell_exec
    else if(function_exists('shell_exec'))
    {
        $output = shell_exec($command) ;
    }

    else
    {
        $output = 'Command execution not possible on this system';
        $return_var = 1;
    }

    return array('output' => $output , 'status' => $return_var);
}

$o = terminal('ls');
if($status == 0)
{
    echo $o['output'];
}
else
{
    //some problem
}