title: PHP: is_array on $arr['key'] with non existing 'key'
body: <p>One of my colleges seem to have an 'undefined index' error on a code I wrote  </p>
  
  <p>This code of mine looks like this:</p>
  
  <pre><code>if ( is_array ($arr['key'])) 
  </code></pre>
  
  <p>My intention was to check whether $arr has a key named 'key', and if the value of that key is array itself. Should I do instead: <code>if( isset($arr['key']) &amp;&amp; is_array ($arr['key']))</code> ?</p>
  
  <p>Maybe the following is equivavlent:
  Let's assume $var is not set. Then, will is_array($var) cause an error or will it just return false?</p>
  
  <p>Thank you</p>