Description:
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Strings can be dereferenced to get sub-strings of length 1, eg. 'hello'[1] === 'e'.

String variables can be dereferenced, eg. ($x = 'hello') && ($x[1] === 'e')

String constants can't be dereferenced, eg. define('MY_CONST', 'hello') && MY_CONST[1].

Test script:
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<?php
define('MY_CONST', 'hello');
var_dump(MY_CONST[1]);


Expected result:
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'string(1) "e"' on stdout

Actual result:
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Parse error: syntax error, unexpected '[' in /home/chris/test2.php on line 5