Description:
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PHP7 throws an exception if we try accessing a member by an index stored in a referenced variable.
In other words, if we have the index in $var and, by one reason or another, make a reference to it (like $ndx2 =& $ndx) we no longer can use it to access a SplFixedArray. (We would have to copy $ndx, or $ndx2, to a temp normal variable and use it instead.)

Test script:
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<?php

$list = new SplFixedArray(10);
$ndx = 1;
$ndx2 =& $ndx;
$list[$ndx] = 123;	// This throws an exception;
$list[$ndx2] = 123;	// as does this, to.
echo 'ok';


Expected result:
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It should just print "ok".

Actual result:
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"Uncaught RuntimeException: Index invalid or out of range"