Php Clear Text Printed To Webpage

I have a html/php script which prints a text string directly to the page. However when I refresh the page that text is still there. I've googled this and check this forum too and t

Solution 1:

if (preg_match($snregex, $sn, $matches)) {
 <command>
} else {
  //echo "Invalid SN. Please re-enter!";
 }

try commenting the echo and check if the text is still there.

if not, every time the else loop gets executed and prints the statement.

solution:

if(isset($_POST['submit']) && (preg_match($snregex, $sn, $matches))
{
 <command>
} else {
  echo"Invalid SN. Please re-enter!";
 }

Solution 2:

Is it printing out from a session or post var that's not getting cleared ? is the input field something like

input value="<?phpif(isset($_POST['serial_number'])){echo S_POST['serial_number'];}?>"