How can I access data on a textbox using $_SERVER['PHP_SELF'] from another page

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PHP

With ?php echo $editFormAction/ $_SERVER['PHP_SELF'] as form action, how can I access
the data entered in a form textbox but that has not been submitted into mysql database from another child window. Thank you

What I have tried:

I have tried Class of Room: <?php echo $_POST['Class_of_Room']; ?>

Room Number: <?php echo ($_POST['Room_Number']); ?>

Customer Name: <?php echo ($_POST['Customer_Name']); ?>

Customer Address: <?php echo ($_POST['Customer_Address']); ?>

Phone number: <?php echo ($_POST['Phone_number']); ?>

Occupation: <?php echo ($_POST['Occupation']); ?>

Amount: <?php echo($_POST['Amount']); ?>.

Check in Time: <?php echo($_POST['Check_in_Time']); ?>

Date: <?php echo ($_POST['Date']); ?>.

Duration: <?php echo ($_POST['Duration']); ?>

Name of Receptionist: <?php echo ($_POST['Name_of_Receptionist']); ?