What are some common techniques for checking if a key already exists in a PHP array before adding a new value?

When adding a new value to a PHP array, it is important to check if the key already exists to avoid overwriting existing data. One common technique is to use the `array_key_exists()` function, which checks if a specific key exists in an array. Another approach is to use the `isset()` function to determine if a key is set and not null in the array. Both methods can help prevent unintentional data loss when adding new values to an array.

// Check if key exists using array_key_exists()
if (!array_key_exists($key, $array)) {
    $array[$key] = $value;
}

// Check if key exists using isset()
if (!isset($array[$key])) {
    $array[$key] = $value;
}