HTML
<form method="post"> ... <input type="submit" name="prim" value="Применить" /> <input type="submit" name="clean" value="Очистить" /> </form>
PHP
if (isset($_POST['prim'])) { ... } else if (isset($_POST['clean'])) { ... }

рис.1.
HTML
<form method="post"> ... <input type="submit" name="prim" value="Применить" /> <input type="submit" name="clean" value="Очистить" /> </form>
PHP
if (isset($_POST['prim'])) { ... } else if (isset($_POST['clean'])) { ... }
рис.1.