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This can be done by array.splice() More details here

$array_1 = array(
    '0' => 'zero',
    '1' => 'one',
    '2' => 'two',
    '3' => 'three',
  );
  
  $array_2 = array(
    'zero'  => '0',
    'one'   => '1',
    'two'   => '2',
    'three' => '3',
  );


  array_splice($array_1, 3, 0, array('sample_key' => 'sample_value'));

  array_splice($array_2, 3, 0,array('sample_key' => 'sample_value'));

print_r($array_1);
/* Result will be:
Array
(
    [0] => zero
    [1] => one
    [2] => two
    [3] => sample_value
    [4] => three
)
*/
print_r($array_2);

/* Result will be :

Array
(
    [zero] => 0
    [one] => 1
    [two] => 2
    [0] => sample_value
    [three] => 3
)
*/