PHP remember url parameter value using $_SESSION - php

Question

I made a script that shows the value of "school_id" in url parameter.
http://mywebsite.com/mygrade?school_id=00000
I use $_GET['school_id'] to display the ID number.
<?php echo $_GET['school_id']; ?>
But I what I want is if the parameter "school_id" is empty, I want to display the previous data entered.
Example, the user already browse http://mywebsite.com/mygrade?school_id=00000 then he browse http://mywebsite.com/mygrade?school_id= which id has no value. It will still display 00000 which is the previous ID he used.
I used this code below but doesn't work.. :(
<?php
session_start();
$_SESSION['schoo_id'] = $_GET['school_id'];
if ($_GET['school_id'] === null || $_GET['school_id'] == ""){
echo $_SESSION['schoo_id'];
}
else{
$_GET['school_id'];
}
?>
Anyone who get my point and could help me?

Answer 1

I'm going to break this down line by line, please let me know in the comments if I need to explain anything further:
Self explanatory:
<?php
session_start();
There is a typo here:
$_SESSION['schoo_id'] = $_GET['school_id'];
But! Fixing it won't resolve your problem. What happens if $_GET['school_id'] is not defined/blank? Guess what, $_SESSION['school_id'] is now blank. Obviously you don't want this behavior, so you'll want to only set $_SESSION['school_id'] if $_GET['school_id'] is defined
accessing $_GET['school_id'] will throw an E_NOTICE error if it isn't defined, so you'll want to instead check its existence, rather than checking to see if it is null.
if ($_GET['school_id'] === null || $_GET['school_id'] == ""){
Oh, that typo was intended. Why misspell school though? No need! :)
echo $_SESSION['schoo_id'];
What is this doing? Nothing! No echo, nothing. Just accessing a variable and doing nothing with it.
}
else{
$_GET['school_id'];
}
?>
Here's what your code should look like, or at least I believe is what you intend:
<?php
session_start();
if (isset($_GET['school_id']) && $_GET['school_id'] !== ""){
$_SESSION['school_id'] = $_GET['school_id'];
}
// $_SESSION['school_id'] will be guaranteed to be what $_GET['school_id'] is (if set)
// or whatever it was last time it was defined
// always echo it.
echo $_SESSION['school_id'];
?>

Answer 2

<?php
session_start();
if ($_GET['school_id'] === null || $_GET['school_id'] == ""){
echo $_SESSION['schoo_id'];
}
else{
$_GET['school_id'];
$_SESSION['schoo_id'] = $_GET['school_id']; //here set the session
}
?>

Answer 3

I agree with Salman A, the simplest way:
<?php
session_start();
if (is_int($_GET['school_id'])) $_SESSION['school_id'] = $_GET['school_id'];
// further use $_SESSION['school_id'] for your needs.
?>

Answer 4

what you need to do here is save the GET value in SESSION only if it is set for later use so this should work
<?php
session_start();
if (!isset($_GET['school_id']) || $_GET['school_id'] === null || $_GET['school_id'] == ""){
echo $_SESSION['schoo_id'];
}
else{
$_SESSION['schoo_id'] = $_GET['school_id'];
echo $_GET['school_id'];
}
?>

Answer 5

You almost have it.
<?php
session_start();
if (isset($_GET['school_id']) && trim($_GET['school_id']) !== '') {
// its a fair assumption to make that 'school_id' is intended to be an integer,
// however I will not make that assumption on the OP's behalf.
$_SESSION['school_id'] = $_GET['school_id'];
}
if (isset($_SESSION['school_id']) {
echo $_SESSION['school_id'];
}
else {
echo 'have not entered a school id yet';
}
?>