URL : http://www.php.net/manual/fr/language.operators.array.php

The documentation appears to be correct from the English docs. I can't speak for the French 
translation, or its accuracy. However, I will note that the addition operator is left-associative (as 
PHP operators take on precedence and associativity rules mostly from C).

So to say that the expression ($arr1 + $arr2) yields left association, is by definition correct. 
Because $arr1 will be appending all elements in $arr2, that belong to keys which do not exist in 
$arr1.

In your example, the elements of $array1:
    (
     'a' => 1,
     'b' => 2
    )
show up first in the resulting array. Any keys found in $array2 that exist in $array1 (in this case 
'a'), will simply be ignored and the values of keys found in the second operand, which do not exist 
in the first operand, will be appended in the yielding expression.

This can be a bit confusing when phrased as "left is added to right" or "right is added to left". So 
instead it's best to explain this from the perspective of an operator as opposed to the perspective 
of the operands (in a technical documentation).

A left associative operator tells us that the operands are grouped to the left in the absence of parenthesis. This means when we spot the operator we need to group its right-hand operand with its 
left-hand operand when applying the operator.

I hope this might help to disambiguate the technical wording found in the French translation.

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.