<?php
// connexion BDD ....
$term = (isSet($_GET['term']) ? $_GET['term'] : '';
$sql = "SELECT codepostal FROM table WHERE codepostal LIKE '$term%'";
// Exécution requête et stockage résultat dans un array $data
echo json_encode($data); // renvoi du contenu sous format JSON
?>

$sql = "SELECT * 
	FROM coordonnees
	LEFT JOIN dossiers ON  dossiers.num_client= coordonnees.num_client
	WHERE activite like '".$activite."%' AND code_postal like '".$code_postal."%' AND mots_cles like '".$mots_cles."%' AND mots_cles_option like '".$mots_cles_option."%'"; 
  	$resultat = mysql_query($sql, $conn) or die(mysql_error());

<?php
        $conn = db_connect();
        $sql = "SELECT *
        FROM entreprises_coordonnees
        LEFT JOIN entreprises_dossiers ON entreprises_dossiers.num_client=entreprises_coordonnees.num_client
        WHERE activite like '%".$activite."%' AND code_postal like '%".$code_postal."%' AND mots_cles like '%".$mots_cles."%' AND mots_cles_option like '%".$mots_cles_option."%' ORDER BY nom_entreprise ASC";
        $resultat = mysql_query($sql, $conn) or die(mysql_error());
        $result = mysql_query($sql, $conn) or die(mysql_error());

 <link href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/themes/base/jquery-ui.css" rel="stylesheet" type="text/css"/>
  <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.5/jquery.min.js"></script>
  <script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/jquery-ui.min.js"></script>
 
   <script>
  $(document).ready(function() {
    $("input#autocompletedep").autocomplete({
    source: [<?php             
mysql_connect("localhost","root","");

/* selection de la base de donnée mysql */
mysql_select_db("societe");
                $sql = mysql_query("SELECT DISTINCT code_postal FROM  entreprises_coordonnees");
                while ($off = mysql_fetch_assoc($sql)) {
                echo '"'.$off['code_postal'].'",';
                }
                mysql_close(); ?>]
});
  });
  </script>

<div class="titre_b">
<b><u>Activit&eacute;</u></b>
<font style="font-size:9px; margin-left:10px;"><b><?php echo CHAMPS_OBLIGATOIRES; ?> (*)</b></font><br /><br />
  <form action="" class="tableau_search" method="get">
        <table align="center" border="0" cellspacing="1" cellpadding="1" width="100%">
        <tr>
                <td width="26%">Activit&eacute; (*) : </td>
                <td width="74%"><input id="autocomplete"  type="text" size="30" name="activite"  maxlength="50" class="inputText" /></td>
        </tr>
        <tr>
          <td>Mot cl&eacute;(s) : </td>
          <td><input type="text" size="30" name="mots_cles" maxlength="50" class="inputText"   /></td>
          </tr>
        <tr>
                <td>D&eacute;partement : </td>
                <td><input type="text" size="3" name="code_postal" maxlength="5" class="inputTextLittle" id="autocompletedep"  /></td> 
        </tr>
        <tr>
                <td>&nbsp;</td>
        </tr>
        <tr>
    <td>&nbsp;</td>
                <td colspan="2" align="left"><input type="submit" value="<?php echo RECHERCHER; ?>"></td>
        </tr>
        </table>
  </form>
  </div>