PHP - problema con contraseña aleatoria

<?php

$conexion = pg_connect("host=localhost port=5432 dbname=pediatria user=postgres password=movilnet");

   error_reporting(E_ALL ^ E_WARNING ^ E_NOTICE);

$email = $_POST['email'];

$e_mail="SELECT * FROM usuarios WHERE email = '$email'";

$res = @pg_query($conexion,$e_mail);

  if(@pg_num_rows($res)>0)

  {

     $cadena = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';

  	 $passw ='';

	 $lng_cadena = strlen($cadena);

	 $longitud = 8;

	 	for($x=1;$x<=$longitud;$x++)

		{

		   $aleatorio = mt_rand(0,lng_cadena-1);

		   $passw = substr($cadena,$aleatorio,1);

		   echo $passw;

		}

   }

   else

   {

   	 echo' <script language="javascript">alert("Atencion, su correo no existe");</script> ';

     echo "<script>location.href='passw.php'</script>";

   }

?>