Jeden asi ze 127 442 způsobů, jak to udělat :

$textsmile = array(':-)', ':-(');
$graphicsmile = array('<img src="smile1.gif">', '<img src="smile2.gif">');


require_once ('../../admin/mysql/mysql.php');

$dotaz = "SELECT *, DATE_FORMAT(datum_prispevku,'%e. %c. %Y v %k:%i') AS datum FROM minichat ORDER BY id_prispevku DESC";

$vysledek = mysql_query ($dotaz);
while ($radek = mysql_fetch_array ($vysledek, MYSQL_ASSOC)) {
 $text = $radek['obsah_prispevku'];
$text = str_replace($textsmile, $graphicsmile, $text);
  echo "<div title='{$radek['datum']}'><div id='autor_chat'><b>{$radek['autor_prispevku']}</b><br/><img src='../../images/chat_pod.jpg' border='0'></div><div>{$text}</div><br/></div>";
  }
  
  mysql_close();