How To repair an undefined array key

 To repair an undefined array key, you can use the isset() function to check if the key is defined before trying to access its value. If the key is not defined, you can assign a default value to it using the = operator. Here is an example:

$arr = [ 'a' => 1, 'b' => 2 ];

if (!isset($arr['c'])) {
    $arr['c'] = 3;
}

Alternatively, you can use the array_key_exists() function, which behaves similarly to isset() but works with any expression, not just variables.

$arr = [ 'a' => 1, 'b' => 2 ];

if (!array_key_exists('c', $arr)) {
    $arr['c'] = 3;
}

You can also use the ternary operator to set the default value if the key does not exist.

$arr = [ 'a' => 1, 'b' => 2 ];
$arr['c'] = isset($arr['c']) ? $arr['c'] : 3;

It's also worth noting that, in PHP 7.4, you can use the null coalesce operator to achieve the same thing in a more concise way:

$arr = [ 'a' => 1, 'b' => 2 ];
$arr['c'] = $arr['c'] ?? 3;