Why is this syntax valid?

Question

I've answered a question today that had a simple solution, but I've yet to understand why it was something for which the PHP interpreter didn't throw a syntax error. So my question is simple:

Why are the statements below considered to have valid syntax?

SomeIdentifier:;
AnythingGoesApparently:;
ThisCanGoOnAndOn:;

Even more so since those identifiers are not defined anywhere.

---

To my knowledge the colon : is only used as part of two operators: The Scope Resolution Operator, The Ternary Operator, but not on its own. It's also used in the Alternative syntax for control structures. But none of these would qualify in this case, so this is bugging me like crazy.

Answer 1

I believe these would be interpreted as goto labels.

For example:

$x = 0;
goto ThisIsBad;
$x++;

ThisIsBad:
$x += 2;

echo $x;

Output: 2

Reference: http://php.net/manual/en/control-structures.goto.php