$dossier = 'images_upload';

$adresse_racine = (substr($_SERVER['DOCUMENT_ROOT'],-1) == '/')? $_SERVER['DOCUMENT_ROOT'] : $_SERVER['DOCUMENT_ROOT'].'/' ;

$adresse_repertoire = $adresse_racine.$dossier.'/';

// pour vérif
echo $adresse_repertoire;

//Ensuite la classe vérifie avec is_dir();
if(!is_dir($adresse_repertoire)) echo 'chemin du dossier invalide'; else echo 'chemin du dossier valide';

<?php
$upload_path = $_SERVER['DOCUMENT_ROOT']."/";
$upload_url = "http://".$_SERVER["HTTP_HOST"]."/";

$upload_extensions = array( 'jpg' , 'jpeg' , 'gif' , 'png' );
$upload_maxsize = 1024*1024; 


function get_pathinfo($path)
{
	$tbl=pathinfo($path);
	if (!isset($tbl["filename"])) $tbl["filename"] = basename($tbl["basename"],".".$tbl["extension"]);
	return $tbl;
}	

function upload_check_extension($file)
{
	global $upload_extensions;
	$fileinfo = get_pathinfo($file);
	return in_array(strtolower($fileinfo["extension"]),$upload_extensions);
}

function upload_check_size($filesize)
{
	global $upload_maxsize;
	return ($filesize <= $upload_maxsize);
}


foreach ($_FILES["files"]["error"] as $key => $error) 
{
    $tmp_name = $_FILES["files"]["tmp_name"][$key];
    $name = $_FILES["files"]["name"][$key];
	$msg = "Ok";
	$url = "";
	$newname = $name;
	$size = $_FILES["files"]["size"][$key];

    if ($error == UPLOAD_ERR_OK) 
	{
		if (!upload_check_extension($name))
		{
			$msg="Erreur : extension non valide";
		}
		else
		{
			if (!upload_check_size($size))
			{
				$msg="Erreur : taille trop importante";
			}
			else
			{		
				if (!move_uploaded_file($tmp_name, $upload_path.$newname)) 
				{
					$msg="Erreur : fichier bien reçu mais impossible à déplacer vers sa destination finale";
				}
				else
				{
					$url = $upload_url.$newname;
				}	
			}
		}
	}	
	else
	{	
		$msg = "Erreur lors du transfert : ".$error;
	}	
	
	if ($tmp_name)
	{
		printf('<tr><td>%s</td><td>%s</td><td>%s</td><td>%s</td></tr>',
			($url) ? "<a href=\"".$url."\" target=\"_blank\">".$url."</a>" : "",
			$name,
			$size,
			$msg);
							
			$sql = 'INSERT INTO photos VALUES("","'.$_POST['continent'].'","'.$_SESSION['id'].'","'.$_POST['nom'].'","'.$name.'")';
			$reponse=$pdo->prepare($sql);
			$reponse->execute(); 
	
	}		
}
?>