PHP
Reading from a session
Learn to read data from $_SESSION and check existence with isset().
Reading data from $_SESSION
You read session data with $_SESSION['key']. But if the key is missing, PHP raises a warning. To avoid this, check existence with isset().
If isset($_SESSION['lang']) returns true, the key is defined and safe to read. If false, it is undefined; use a default value.
Tip: $_SESSION['lang'] ?? 'en' (null coalescing) gives you both the check and a default in one line.
Defaults for missing keys
When reading from a session, a key may be undefined. Two common approaches: an if/else with isset(), or the null coalescing operator ??.
$lang = isset($_SESSION['lang']) ? $_SESSION['lang'] : 'en'; is the classic ternary form. Shorter: $lang = $_SESSION['lang'] ?? 'en';
Both do the same: if undefined, return 'en'; otherwise the stored value. The short form is preferred in modern PHP 8.
Exercise Read a default language from a missing key
Call session_start(). With if/else, check isset($_SESSION['lang']): if defined, assign the stored value to $lang; otherwise assign 'en'. Then echo $lang.
Show example solution
Call session_start, use if(isset($_SESSION['lang'])) to set $lang from the session, else 'en', then echo $lang.
Example solution:
<?php
// Print the default language if $_SESSION['lang'] is missing
session_start();
if (isset($_SESSION['lang'])) {
$lang = $_SESSION['lang'];
} else {
$lang = 'en';
}
echo $lang . "\n";
Test yourself
-
Complete the isset key check
<?php
session_start();
$_SESSION['count'] = 5;
if (____ && $_SESSION['count'] > 0) {
echo 'ok';
} else {
echo 'en';
}
Show answer
<?php
session_start();
$_SESSION['count'] = 5;
if (isset($_SESSION['lang']) && $_SESSION['count'] > 0) {
echo 'ok';
} else {
echo 'en';
}
-
What does isset($_SESSION['missing']) return?
Show answer
false — Correct. When the key is undefined, isset returns false.
-
Match the session reading tools
- isset($_SESSION['k'])
- Checks if key exists
- $_SESSION['k']
- Reads the value
- ?? 'default'
- Gives default if undefined
- empty()
- Checks if empty
Next lesson: unset() and session_destroy()
