ขอถามความแตกต่างระหว่าง if( isset($_POST["add_prd"])) กับ if($_REQUEST["flag"]==1)

	if(var_dump($_POST)){
	process_form();
}
else
	show_form();

  <?
  var_dump($_POST["add_prd"]);
if (isset($_POST["add_prd"])){
//	if($_REQUEST["flag"]==1){
	
	process_form();
}
else
	show_form();
	
function show_form(){ ?>
<form id="form1" name="form1" method="post" action="<? $_SERVER['PHP_SELF']?>">
 <br /> 
 <input type="hidden" name="flag" value="1">
 Model: <input type="text" name="fr_model" id="fr_model" /><br /><br />
  S/N:<input type="text" name="fr_sn" id="fr_sn" /><br /><br />
  <input type="submit" name"add_prd" value="เพิ่มข้อมูล">
</form>

<?php
	var_dump($_POST["add_prd"]);
	if (isset($_POST["add_prd"])){
	//	if($_REQUEST["flag"]==1){
		process_form();
	}
	else
		show_form();
?>
    
<?php function show_form(){ ?>
    <form id="form1" name="form1" method="post" action="<?php echo htmlspecialchars($_SERVER['PHP_SELF'])?>">
     <br /> 
     <input type="hidden" name="flag" value="1">
     Model: <input type="text" name="fr_model" id="fr_model" /><br /><br />
      S/N:<input type="text" name="fr_sn" id="fr_sn" /><br /><br />
      <input type="submit" name="add_prd" value="add">
    </form>
<?php }?>

<?php function process_form(){ ?>
	<?php echo 'hello world';?>
<?php }?>