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copied!<p>At work I've got a VM instance of PHP that includes a PECL extension called VLD. This lets you execute PHP code from the commandline and rather than execute it, it returns the generated opcode instead. </p> <p>It's brilliant at answering questions like this. </p> <p><a href="http://pecl.php.net/package/vld" rel="noreferrer">http://pecl.php.net/package/vld</a></p> <p>Just in case you go this route (and if you're generally curious about how PHP works internally, i think you should) you should definitely install it on a virtual machine (that is, i wouldn't install it on a machine i'm trying to develop on or deploy to). And this is the command you'll use to make it sing: </p> <pre><code>php -d vld.execute=0 -d vld.active=1 -f foo.php </code></pre> <p>Looking at the opcodes will tell you a more complete story, however, I have a guess.... Most of PHP's built-ins make a copy of an Array/Object and act upon that copy (and not a copy-on-write either, an immediate copy). The most widely known example of this is foreach(). When you pass an array into foreach(), PHP is actually making a copy of that array and iterating on the copy. Whis is why you'll see a significant performance benefit by passing an array as a reference into foreach like this: </p> <p>foreach($someReallyBigArray as $k => &$v)</p> <p>But this behavior -- that passing in an explicit reference like that -- is unique to foreach(). So I would be very surprised if it made an array_key_exists() check any faster. </p> <p>Ok, back to what I was getting at.. </p> <p>Most the built-ins take a copy of an array and act upon that copy. I am going to venture a completely unqualified guess that isset() is highly optimized and that one of those optimizations is perhaps to not do an immediate copy of an Array when its passed-in. </p> <p>I'll try to answer any other questions you may have but you could probably read a lot of you google for "zval_struct" (which is the data structure in the PHP internals which stores each variable. It's a C struct (think.. an associative array) that has keys like "value", "type", "refcount". </p>

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