<?php echo "$array[index]"; ?>

works while

<?php echo "$array['index']"; ?>

gives

PHP Parse error:  parse error, expecting `T_STRING' or `T_VARIABLE' or `T_NUM_STRING' in - on line 1

the error message is not very helpfull (knowing the exact character position in the line would help a little bit, but that's another story) and the different syntax requirements for array indices within and outside of strings are somtimes confusing for newbies, too

so "$array['index']" should be supported by the parser

PS: i know about "{$array['index']}", but that is not the point ...