$tab=array();
$tabA = explode("\n",file_get_contents("A.txt");
$tabB = explode("\n",file_get_contents("B.txt");
foreach($tabA as $i => $row)
{
$tabtmpA = explode(";",$row);
if(in_array($tabtmpA[2],$tabB))
	$tab[] = $row;
}
$str = implode("\n",$tab);
file_put_contents($str,"fichier.txt");