This fails on

• ships touching corners
• a board that contains 6 ships with size 1 and 2 ships with size 2 (one 2-ship split into two); incorrectly counts "the right number of ship squares"

Otherwise, pretty concise!

A comment is useful if it tells us something the code cannot express.
This comment is incorrect (it says "add", whereas the code performs "multiply") and even if it were correct, it is not useful.

While this code may work, it is no pleasure maintaining code like this
One statement, one line; use meaningful variable names--to start somewhere.

On one end, there is Primitive Obsession. On the other end, there is this: reeks of speculative generality.

Your method comments add absolutely nothing. Comment only what the code cannot say.
'formatTime' is a misleading name (the time is not formatted). twoDigits() may be a suitable alternative.

Please do not leave echo statements in your method.
Look into:

• division instead of repeated subtraction;
• number formatting functions for sprintf()

No, that is not correct. Check the first conjunct inside the if().
Your reply does point out that this code is not optimally readable.

The array $solutions used inside in_array is constructed by concatenating the inputs into a string.

merely incrementing the test factor $i is very inefficient; you're trying all even numbers

+1 for upper limit (sqrt n) on factor