PHP Session Variable

I am creating a session variable.  $partner_id = $_SESSION[partner];  I'm using it on my pages. It is set when a customer enters the site through a specific landing page.  However, if the customer does not enter the site through that specific landing page I get an error, unknow index "partner".  How do I fix that?
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rgranlundAsked:
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Dave BaldwinFixer of ProblemsCommented:
First, index names should be quoted or else they may be mistaken for constants.  Here is the code I use for such things.  $partner_id must be set to something or else it becomes an error itself.
if (isset($_SESSION["partner"])) {$partner_id = $_SESSION["partner"]; }
else $partner_id = ''; ;

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Ray PaseurCommented:
This is almost certainly not the right line of code:

$partner_id = $_SESSION[partner];

You can learn about quotes here:
http://www.experts-exchange.com/Web_Development/Web_Languages-Standards/PHP/A_12241-Quotation-Marks-in-PHP.html

How PHP sessions work is here:
http://www.experts-exchange.com/Web_Development/Web_Languages-Standards/PHP/A_11909-PHP-Sessions-Simpler-Than-You-May-Think.html

Dave's solution is good.  There is also ternary notation:
$partner_id = (!empty($_SESSION['partner'])) ? $_SESSION['partner'] : NULL;

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