Keys and values
A PHP array is an ordered map. Keys can be integers or strings. The short syntax ["Ada", "Grace"] creates a list with integer keys starting at zero.
Append a value
$names[] = "Linus"; adds a value at the next integer key. You do not need to calculate the list length first. Appending preserves existing values and can add a duplicate.
A common mistake
Do not assume every array has consecutive integer keys. Removing an entry does not automatically turn all remaining keys into a sequence. Use explicit keys when your data represents a lookup rather than a list.
Tested examples
$names = ["Ada"]; $names[] = "Grace"; echo json_encode($names);Output
["Ada","Grace"]