PHP deleting variable after new form - php

Question

In my code, I have two forms for users to select options. The first variable will save but as soon as the user submits the second form, the variable from the first form is no longer saved.
<div class = "school">
<h3>Please select the university you previously attended</h3>
<form action = "" method = "post" name = "school_form">
<select name="school" size ="10">
<?php
//shows options for $selected_school
$sql = "SELECT DISTINCT school FROM data;";
$result = mysqli_query($conn, $sql);
$resultCheck = mysqli_num_rows($result);
if ($resultCheck > 0){
while($row = mysqli_fetch_assoc($result)){
// inserts all data as array
echo "<option>". $row['school'] ."</option>";
}
}
?>
</select>
<br>
<input type ="submit" name = "submit_school" value = "Enter">
</form>
<?php
//saves selected option as $selected_school
if(isset($_POST['submit_school'])){
$selected_school = mysqli_real_escape_string($conn, $_POST['school']);
echo "You have selected: " .$selected_school;
}
?>
</div>
<div class ="courses">
<h3>Please select the courses you took</h3>
<form action = "" method ="post" name ="course_form">
<?php
//user shown options for courses
$sql2 = "SELECT transfer_course, transfer_title FROM data WHERE school = ? ORDER BY transfer_course ASC";
$stmt = mysqli_stmt_init($conn);
if(!mysqli_stmt_prepare($stmt, $sql2)) {
echo "SQL statement failed";
} else {
mysqli_stmt_bind_param($stmt, "s", $selected_school);
mysqli_stmt_execute($stmt);
$result2 = mysqli_stmt_get_result($stmt);
while($row2 = mysqli_fetch_assoc($result2)){
echo "<input type='checkbox' name ='boxes[]' value = '" . $row2['transfer_course'] . "' >" . $row2['transfer_course'] . "<br>";
}
}
?>
<br>
<input type ="submit" name = "submit_courses" value = "Enter">
</form>
<br>
<?php
//saved selected option(s) as $selected_course
if(isset($_POST['submit_courses'])){//to run PHP script on submit
if(!empty($_POST['boxes'])){
foreach($_POST['boxes'] as $selected_course){
echo "You have selected: " . $selected_course . "</br>";
}
}
}
?>
</div>
<div class = "output">
<h3>Course Equivalency</h3>
<?php
$sql3 = "SELECT arcadia_course, arcadia_title FROM data WHERE school = " . $selected_school . " AND transfer_course = " . $selected_course . "";
$result3 = mysqli_query($conn, $sql3);
if($result3)
{
while($row3 = mysqli_fetch_assoc($result3)){
echo $row3['arcadia_course'] . " " . $row3['arcadia_title'] . "<br>";
}
} else {
echo "failed";
echo $sql3;
}
?>
So by the time I get to my next sql statement
$sql3 = "SELECT arcadia_course, arcadia_title FROM data WHERE school = " . $selected_school . " AND transfer_course = " . $selected_course . "";
When the school is selected, it saves the variable, but when the course is selected, $selected_school becomes blank again.
I already have session_start() at the top of the page.

Answer 1

You can used session variable ,it will help to make data accessible across the various pages .
So,whenever form get submitted you can save that value in session and retrieve it anytime.In top of your php file you need to start session i.e session_start(); .Then in your code
<?php
//saves selected option as $selected_school
if(isset($_POST['submit_school'])){
$_SESSION['selected_school ']=$selected_school;// here you are storing value to session
$selected_school = mysqli_real_escape_string($conn, $_POST['school']);
echo "You have selected: " .$selected_school;
}
?>
Same you can do with your $selected_course also .Then you can passed value to your query like below :
$sql3 = "SELECT arcadia_course, arcadia_title FROM data WHERE school = " .$_SESSION['selected_school ']. " AND transfer_course = " .$_SESSION['selected_course']. "";
For more information refer here

Answer 2

It looks like your option doesn't have a value it is passing. Try this in your first form while loop:
echo '<option value="' . $row['school'] . '">' . $row['school'] . '</option>';
It looks like there may be some more issues you are having as well. If this doesn't fix your issue, I'll dig deeper.
EDIT: Then, yes, as others have suggested, you probably want to add a hidden input field to pass that variable value on the second form submit as well.
What we are saying about the hidden input field is this:
<input type="hidden" name="selected_school" value="<?php if(isset($selected_school) echo $selected_school; ?>">