Final answer:
In PHP, array keys are not automatically assigned for all elements; developers have the option to either specify keys explicitly or let PHP generate numeric indices when keys are not specified. PHP utilizes the next available integer index if no key is provided, and if the array contains numeric keys, PHP will select the integer that is one greater than the largest present numeric key.
Step-by-step explanation:
In PHP, array keys are not always assigned automatically. While PHP does automatically assign numeric indices to array elements that do not specify a key, developers also have the option to explicitly define keys. When you create an array, you can define both the keys and the values. For example, using the array() function or the shorthand array syntax, you can specify the key for each element:
$array = array(
'key1' => 'value1',
'key2' => 'value2'
);
// or using the shorthand syntax
$array = [
'key1' => 'value1',
'key2' => 'value2'
];
If you do not specify keys when adding elements to an array, PHP will use the next available integer index starting from 0. For instance:
$array[] = 'value1'; // assigned key 0
$array[] = 'value2'; // assigned key 1
However, if an array already has elements and the largest key is an integer, PHP will use the integer one larger than the maximum key already present in the array:
$array = [
3 => 'value1',
'foo' => 'value2'
];
$array[] = 'value3'; // assigned key 4, not 0 or 2