Random set of images using php

I have this code that I've been using to select one random image. Now I need to select four random images.

I've tried to modify the code and it does work but I can't figure out a way to prevent the same images appearing twice. My knowledge of php is basic at best.

Can anyone shed any light please?

Thanks

my code

<?php
$root = $_SERVER['DOCUMENT_ROOT'];
$path = '/_/images/banners/';

function getImagesFromDir($path) {
    $images = array();
    if ( $img_dir = @opendir($path) ) {
        while ( false !== ($img_file = readdir($img_dir)) ) {

            if ( preg_match("/(\.gif|\.jpg|\.png)$/", $img_file) ) {
                $images[] = $img_file;
            }
        }
        closedir($img_dir);
    }
    return $images;
}

function getRandomFromArray($ar) {
    mt_srand( (double)microtime() * 1000000 );
    $num = array_rand($ar);
    return $ar[$num];
}

$imgList = getImagesFromDir($root . $path);

$imgA = getRandomFromArray($imgList);
$imgB = getRandomFromArray($imgList);
$imgC = getRandomFromArray($imgList);
$imgD = getRandomFromArray($imgList);
?> 

<img src="<?php echo $path . $imgA ?>" alt="<?php echo ucfirst(preg_replace('/\\.[^.\\s]{3,4}$/', '', $imgA)) . ' Logo'; ?>">
<img src="<?php echo $path . $imgB ?>" alt="<?php echo ucfirst(preg_replace('/\\.[^.\\s]{3,4}$/', '', $imgB)) . ' Logo'; ?>">
<img src="<?php echo $path . $imgC ?>" alt="<?php echo ucfirst(preg_replace('/\\.[^.\\s]{3,4}$/', '', $imgC)) . ' Logo'; ?>">
<img src="<?php echo $path . $imgD ?>" alt="<?php echo ucfirst(preg_replace('/\\.[^.\\s]{3,4}$/', '', $imgD)) . ' Logo'; ?>">