PHP - What does "Operator precedence and associativity do not specify an order of evaluation" mean? -


i reading operator precedence section of php manual. confused (or say, don't understand following sentences quite much):

operator precedence , associativity determine how expressions grouped, not specify order of evaluation. php not (in general case) specify in order expression evaluated , code assumes specific order of evaluation should avoided, because behavior can change between versions of php or depending on surrounding code.

it gives 2 examples illustrate undefined order of evaluation.

<?php $a = 1;  echo $a + $a++; // may print either 2 or 3 >?

from understand, $a evaluats 1 first because associativity of addition operator left. 1 added $a++, evaluates 1. so, result should 2. why comment in documentation "may print either 2 or 3"?

the second example is:

<? $i = 1; $array[$i] = $i++; // may set either index 1 or 2 ?>

similarly, $i++ evaluates 1 first because associativity of assignment operator right. value of 1 should set index 2 of array. why comment "may set either index 1 or 2"?

the explanation can think of order of code in 2 examples above can executed opposite of reasoned.

any thoughts me unravel confusion appreciated.

just because operator left-associative, doesn't mean parameters evaluated left-to-right. in

$a + $a++

it can evaluate $a first or $a++ first. in first case, result 1 + 1 = 2; in second case result 2 + 1 = 3.

associativity specifies how multiple expressions grouped when they're combined. instance, if have

1 - 2 + 3

left-associativity specifies interpreted as

(1 - 2) + 3 = 2

rather than

1 - (2 + 3) = -4

but if expression had sub-expressions side effects, still evaluate them right-to-left (or in mixed order).


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