$t = "SELECT count(*) AS `count` FROM `messages` WHERE `sender`='id_user' OR `addressee`='id_user' GROUP BY `sender`+`addressee`";
$lines_count = mysql_query ( $t ),
 
$lines_count = mysql_result ($lines_count, 0);

$t = "SELECT * FROM `messages` WHERE `sender`='id_user' OR `addressee`='id_user' GROUP BY `sender`+`addressee`";
$lines_count = mysql_query ( $t ),
 
$lines_count = mysql_num_rows($lines_count);

$t = "SELECT * FROM `messages` WHERE `sender`='id_user' OR `addressee`='id_user' GROUP BY `sender`+`addressee` ORDER BY `id` ASC" LIMIT 0, 15;
$result= mysql_query ( $t ),

for ( $i = 0; $i < mysql_num_rows ( $result ); $i++ ) {
     $messages_list = mysql_fetch_assoc ( $result );

}

SELECT count(DISTINCT `sender`+`addressee`) FROM `messages`
WHERE 1 IN (`sender`,`addressee`)

SELECT count(DISTINCT `sender`+`addressee`) FROM `messages`
WHERE 1 IN (`sender`,`addressee`)

$t="SELECT count(DISTINCT `sender`+`addressee`) FROM `" . DB_PREFIX . "messages`
WHERE `sender`='1' OR `addressee`='1'";
 
$count = mysql_query ( $t );
$count = mysql_result ($count, 0);