PHP: create an array

<?php
// create an empty array
$a1 = array();
/*
array(0) {
}
*/

// create an array with the values 1, 3 and 5 (index generated automatically, starting at 0)
$a2 = array(1, 3, 5);
/*
array(3) {
  [0]=>
  int(1)
  [1]=>
  int(3)
  [2]=>
  int(5)
}
*/

// create an array by specifying indexes
$a3 = array(1 => 'a', 2 => 'b', 3 => 'c');
/*
array(3) {
  [1]=>
  string(1) "a"
  [2]=>
  string(1) "b"
  [3]=>
  string(1) "c"
}
*/

// add an element to an array without specifying index
$a2[] = 7;
/*
array(4) {
  [0]=>
  int(1)
  [1]=>
  int(3)
  [2]=>
  int(5)
  [3]=>
  int(7)
}
*/

// add an element to an array at the specified index
$a3[4] = 'd';
/*
array(4) {
  [1]=>
  string(1) "a"
  [2]=>
  string(1) "b"
  [3]=>
  string(1) "c"
  [4]=>
  string(1) "d"
}
*/