SOLUTION 1

As you probably already know, session_start is an in-built PHP function that will start a new session.

The problem is:

• If you call it more than once, your script will throw an E_NOTICE error.
• Although the solution seems pretty straight forward (don’t call it more than once), in certain scenarios, you won’t be entirely sure if a session has already been started or not.
• In some cases, it might be out of your control.
• There are two ways to approach this.

If you are using a PHP version that is lower than 5.4.0, you can do the following:

<?php
if(session_id() == ''){
    //session has not started
    session_start();
}
 
var_dump(session_id());

• If you run the code above, you’ll see that a session is always started. This is because:
• We check to see if the function session_id returns an empty string.
• If session_id returns an empty string, we can conclude that the session has not been started yet.
• If this is the case, we can start the session by calling the function session_start.
• In PHP version 5.4.0 and above, we can make use of the function session_status, which returns the status of the current session.
• This function can return three different integer values, all of which are available as predefined constants.

These are:

<?php
if(session_status() == PHP_SESSION_NONE){
    //session has not started
    session_start();
}

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As you can see, using this function makes your code a little more self-explanatory!

Note that you can also check to see if the $_SESSION array has been set. However, it is worth noting that $_SESSION can be manually initialized like so:

<?php
$_SESSION = array(
    'test' => true
);

i.e. The $_SESSION array might exist, even if a session hasn’t already been started.

SOLUTION 2

if (version_compare(phpversion(), '5.4.0', '<')) {
     if(session_id() == '') {
        session_start();
     }
 }
 else
 {
    if (session_status() == PHP_SESSION_NONE) {
        session_start();
    }
 }

SOLUTION 3

Prior to PHP 5.4 there is no reliable way of knowing other than setting a global flag.

Consider:

var_dump($_SESSION); // null
session_start();
var_dump($_SESSION); // array
session_destroy();
var_dump($_SESSION); // array, but session isn't active.

session_id(); // returns empty string
session_start();
session_id(); // returns session hash
session_destroy();
session_id(); // returns empty string, ok, but then
session_id('foo'); // tell php the session id to use
session_id(); // returns 'foo', but no session is active.

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SOLUTION 4

Sample code:

if (!isset($_SESSION)) session_start();