是否有必要在追加数组之前定义它？

Question

Trivial question.

Suppose I have the following code

<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);

$words = ['Apple', 'Avocado', 'Banana', 'Blueberry'];

$dict = [];
// build a dictionary keyed on the first letter
foreach ($words as $word) {
    $letter = $word[0];
    // is this condition necessary?
    if (!isset($dict[$letter])) {
        $dict[$letter] = [];
    }
    $dict[$letter][] = $word;
}
?>

Usually when I build a dictionary, before appending an entry, I ensure that the array exist as shown in my example.

I always thought a warning would show up otherwise, however it does not seem to be the case.

So is the IF condition necessary?

Answer 1

From the official documentation on arrays (emphasis mine):

$arr[key] = value;

$arr[] = value;

// key may be an integer or string

// value may be any value of any type

If $arr doesn't exist yet, it will be created, so this is also an alternative way to create an array. This practice is however discouraged because if $arr already contains some value (e.g. string from request variable) then this value will stay in the place and [] may actually stand for string access operator. It is always better to initialize a variable by a direct assignment.

No warning will be produced, but for clarity's sake it is preferred to initialize by direct assignment as you've always been doing.