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Date: Wed, 24 May 2006 10:49:43 -0700
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Subject: Re: left/right order of assignment and ++ increment changed? 5.0.x -->5.1.x
From: pollita@php.net ("Sara Golemon")

> Seems the order or left vs right assignment evaluation has changed somehow 
> recently in my upgrade to PHP 5.1.4.  See the following code:
>
That's is indeed a quirk of a change in the way variables are 
retreived/stored between 5.0 and 5.1.

Is it an unexpected BC break? In functional terms yes, but technically no. 
As mike already replied, using the same var more than once in a single 
expression where the value of the variable is expected to change is (and has 
always been) considered "undefined behavior".  I'd recommend changing your 
statement to:
$data[$index] = $index;
++$index;

And before you complain about the "extra work" for the engine, let me just 
mention this is still lighter weight than what 5.0 and 4.x were doing.

-Sara