Officially, the behaviour is undefined. The following documents what actually happens. It boils down to the fact that pre-increments and pre-decrements leave the input variable on the stack while post-increments and post-decrements leave a copy of it.

$a++ + ++$a
                                  $a  Stack
1. Push copy of $a on stack       10  10
2. Increment $a                   11  10
3. Increment $a                   12  10
4. Push $a on stack               12  10,$a
5. Add                            12  22
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++$a + $a++
                                  $a  Stack
1. Increment $a                   11
2. Push $a on stack               11  $a
3. Push copy of $a on stack       11  $a,11
4. Increment $a                   12  $a,11
5. Add                            12  23
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Note: These steps don't correspond one-for-one to the ops actually involved for the sake of simplicity.