Here's the PHP script generating the error :

<?php

echo "<h3>LDAP query test</h3>";
echo "Connecting ...";
$ds=ldap_connect("nwgwia01");  // must be a valid LDAP server!
echo "connect result is ".$ds."<p>";

if ($ds) {
    $r=ldap_bind($ds);     // this is an "anonymous" bind, typically
                           // read-only access
    echo "Bind result is ".$r."<p>";

    // Search surname entry
    $sr=ldap_search($ds,"O=ERAM", "mail=*");
    echo "Search result is ".$sr."<p>";

    echo "Number of entires returned is ".ldap_count_entries($ds,$sr)."<p>";

    echo "Getting entries ...<p>";
    $info = ldap_get_entries($ds, $sr);
        $T->get_array_elems($info);

    echo "Closing connection";
    ldap_close($ds);

} else {
    echo "<h4>Unable to connect to LDAP server</h4>";
}
?>        

The LDAP Server is a Novell Groupwise 5.5 system.
The ldap_search command work fine if search on "sn=" or "givenname=".
BUT it generate the following warning message when used with "mail=..."
Warning: LDAP: Unabel to perform the search: Erreur d'operation in <php_script> line 14.

Ideas ?