session - PHP remember url parameter value using $_SESSION -

i made script shows value of "school_id" in url parameter.

http://mywebsite.com/mygrade?school_id=00000 

i use $_get['school_id'] display id number.

<?php echo $_get['school_id']; ?> 

but want if parameter "school_id" empty, want display previous data entered.

example, user browse http://mywebsite.com/mygrade?school_id=00000 browse http://mywebsite.com/mygrade?school_id= id has no value. still display 00000 previous id used.

i used code below 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 point , me?

i'm going break down line line, please let me know in comments if need explain further:

self explanatory:

<?php session_start();  

there typo here:

$_session['schoo_id'] = $_get['school_id']; 

but! fixing won't resolve problem. happens if $_get['school_id'] not defined/blank? guess what, $_session['school_id'] blank. don't want behavior, you'll want only set $_session['school_id'] if $_get['school_id'] defined

accessing $_get['school_id'] throw e_notice error if isn't defined, you'll want instead check existence, rather checking see if null.

if ($_get['school_id'] === null || $_get['school_id'] == ""){ 

oh, typo intended. why misspell school though? no need! :)

    echo $_session['schoo_id']; 

what doing? nothing! no echo, nothing. accessing variable , doing nothing it.

} else{     $_get['school_id']; } ?> 

here's code should like, or @ least believe intend:

<?php session_start();  if (isset($_get['school_id']) && $_get['school_id'] !== ""){     $_session['school_id'] = $_get['school_id']; }  // $_session['school_id'] guaranteed $_get['school_id'] (if set) // or whatever last time defined // echo it. echo $_session['school_id']; ?>