A parse error results when an increment or decrement operator is used in an array index that falls between double quotes. 

For example:
<?
$a = array(1 => "un", "deux", "trois", "quatre", "cinq");
$i = 1;
while ($i < 5) echo "Number $i is $a[$i++]<br />";
?>

Yields -> Parse error: parse error, expecting `']'' in public_html/testarray.php on line 4

While this works fine: 

while ($i < 5) echo "Number $i is ". $a[$i++] ."<br />";

I think this is part of a larger issue of resolving references inside quoted array indexes. It seems an error occurs whenever using an index that's anything more complicated than a literal value or a simple variable. 

For example, given a function returnNumber() that simply returns whatever number you pass to it, this line produces a parse error:

echo "Number 2 is $a[returnNumber(2)]<br />";

While this one prints out the translation for the number 2, as expected:

echo "Number 2 is ".$a[returnNumber(2)]."<br />";