PHP: Avoid undefined index?
<p>Every time a POST value is not equal to the list of values set in an array will return: Undefined Index error, I made an if statement but is not working.</p>

<p>Here's the if statement:</p>

<pre><code> if ($products[$_POST['product']] == $_POST['product']) {
do everything;}
else {
echo "This item is not available";
}
</code></pre>

<p><strong>EDIT2:</strong></p>

<p>Seen the current situation avoiding the warning wont help much because I'm dealing with several factors, for example a list of items in a Shopping Cart, if the invalid product is not removed, it will be added to the shopping list session.</p>

<p>This is the full script:</p>

<pre><code>&lt;?php

session_start();

//Getting the list
 $_SESSION['list'] = isset($_SESSION['list']) ? $_SESSION['list'] : array();    

 //stock    
 $products = array(      
     'Pineaple' =&gt; 500, 'Banana' =&gt; 50, 'Mango' =&gt; 150,       
     'Milk' =&gt; 500, 'Coffe' =&gt; 1200, 'Butter' =&gt; 300,      
     'Bread' =&gt; 450, 'Juice' =&gt; 780, 'Peanuts' =&gt; 800,      
     'Yogurt' =&gt; 450, 'Beer' =&gt; 550, 'Wine' =&gt; 2500,    
 );    

if( isset($_POST['product']) ){


     //Saving the stuff    
     $new_item = array(      
         'item' =&gt; $_POST['product'],       
         'quantity' =&gt; $_POST['quantity'],     
         'code' =&gt; $_POST['code'],      
         'price' =&gt; $products[$_POST['product']] * $_POST['quantity'],    

     );



    $new_product = true;    
    foreach($_SESSION['list'] as $key =&gt; $item) {      
        if ($item['item'] == $new_item['item']) {        
        $_SESSION['list'][$key]['quantity'] += $new_item['quantity'];        
        $_SESSION['list'][$key]['price'] = $products[$new_item['item']] * $new_item['quantity'];        
        $new_product = false;
        }    
    }   

    if ($new_product) {      
        $_SESSION['list'][] = $new_item;        
    }    

    /*if ($new_item['item'] != $products[$new_item['item']]) {
        echo "This item is not available";}*/

    //listing    
    echo  "&lt;b&gt;SHOPPING LIST&lt;/b&gt;&lt;/br&gt;";    
    foreach($_SESSION['list'] as $key =&gt; $item) {       
        echo 'Product .'. $key. ' '. $item['item'], ' ', $item['quantity'], ' units: ', $item['price']. '&lt;br /&gt;';    
        }

}

else {
echo "This item is not available";
}

echo "&lt;/br&gt; &lt;a href='index.html'&gt;Return to index&lt;/a&gt; &lt;/br&gt;";

//Printing session
var_dump($_SESSION);

session_destroy();

?&gt;
</code></pre>
