Interpolation (double quoted string) of Associative Arrays in PHP

When interpolating PHP's string-indexed array elements (5.3.3, Win32) the following behavior may be expected or not:

$ha = array('key1' => 'Hello to me');

print $ha['key1'];   # correct (usual way)
print $ha[key1];     # Warning, works (use of undefined constant)

print "He said {$ha['key1']}"; # correct (usual way)
print "He said {$ha[key1]}";   # Warning, works (use of undefined constant)

print "He said $ha['key1']";   # Error, unexpected T_ENCAPSED_AND_WHITESPACE
print "He said $ha[ key1 ]";   # Error, unexpected T_ENCAPSED_AND_WHITESPACE
print "He said $ha[key1]";     # !! correct (How Comes?)

Inerestingly, the last line seems to be correct PHP code. Any explanations? Can this feature be trusted?

Solution 1:

Yes, you may trust it. All ways of interpolation a variable are covered in the documentation pretty well.

If you want to have a reason why this was done so, well, I can't help you there. But as always: PHP is old and has evolved a lot, thus introducing inconsistent syntax.

Solution 2:

Yes, this is well defined behavior, and will always look for the string key 'key', and not the value of the (potentially undefined) constant key.

For example, consider the following code:

$arr = array('key' => 'val');
define('key', 'defined constant');
echo "\$arr[key] within string is: $arr[key]";

This will output the following:

$arr[key] within string is: val

That said, it's probably not best practice to write code like this, and instead either use:

$string = "foo {$arr['key']}"

or

$string = 'foo ' . $arr['key']

syntax.

Solution 3:

The last one is a special case handled by the PHP tokenizer. It does not look up if any constant by that name was defined, it always assumes a string literal for compatibility with PHP3 and PHP4.