Warrnig

Bonsoir,svp aider moi
signifie quoi cette erreur::

Warning: mysql_num_rows(): 4 is not a valid MySQL result resource in C:\Users\CBS-COMPUTER\Desktop\EasyPHP-5.3.3.1\www\doc1\imprim_fact_veh.php on line 32

la ligne 32 c'est dans cette requete
while($i<mysql_num_rows($req))


voila le code...merci
<?php


//connexion a la base
$connection = mysql_connect("localhost","root","");
if ( ! $connection )
die ("connection impossible");

//On sélectionne la BDD
$mabasededonnee="bdd";
mysql_select_db($mabasededonnee) or die ("pas de connection");

$num_cmd=$_GET["idcmd"];
$req=mysql_query("select * from 'commande' where num_com='$num_cmd'") ;
$i=0;

while($i<mysql_num_rows($req))
{
$ligne=mysql_fetch_row($req);

$foul=mysql_query("select * from 'facture' where num_com='$num_cmd'");
$inn=0;

while ($inn<mysql_num_rows($foul))
{
$frit=mysql_fetch_row($foul);
$inn=$inn+1;}

$r=mysql_query("select * from 'client' where login_client='$ligne[1]'");
$li=mysql_fetch_row($r);



?>