PHP's sequential/associative array behavior appears to depend on whether the array has a continuous, monotonically increasing sequence of indices starting at 0. Unsetting an index in the middle of the sequence will make the array behave associatively. Unsetting out of order indices such that order is restored (e.g. [0,2,1] to [0,1]) will restore…

gistfile1.php

php > $foo = [];
php > $foo[0] = 'zero';
php > $foo[1] = 'first';
php > $foo[0] = 'second';
php > var_dump($foo);
array(2) {
  [0] =>
  string(6) "second"
  [1] =>
  string(5) "first"
}
php >
php > $foo = [];
php > $foo[0] = 'zero';
php > $foo[2] = 'first';
php > $foo[1] = 'second';
php > var_dump($foo);
array(3) {
  [0] =>
  string(4) "zero"
  [2] =>
  string(5) "first"
  [1] =>
  string(6) "second"
}
php > unset($foo[2]);
php > $foo[2] = 'first';
php > var_dump($foo);
array(3) {
  [0] =>
  string(4) "zero"
  [1] =>
  string(6) "second"
  [2] =>
  string(5) "first"
}

php > $foo = [ 'a', 'b', 'c', 'd' ];
php > var_dump($foo);
php > $foo[4] = 'e';
php > var_dump($foo);
array(5) {
  [0] =>
  string(1) "a"
  [1] =>
  string(1) "b"
  [2] =>
  string(1) "c"
  [3] =>
  string(1) "d"
  [4] =>
  string(1) "e"
}
php > unset($foo[1]);
php > var_dump($foo);
array(4) {
  [0] =>
  string(1) "a"
  [2] =>
  string(1) "c"
  [3] =>
  string(1) "d"
  [4] =>
  string(1) "e"
}
php > $foo[1] = 'Z';
php > var_dump($foo);
array(5) {
  [0] =>
  string(1) "a"
  [2] =>
  string(1) "c"
  [3] =>
  string(1) "d"
  [4] =>
  string(1) "e"
  [1] =>
  string(1) "Z"
}
php > unset($foo[1]);
php > array_splice($foo, 1, 0, 'y');
php > var_dump($foo);
array(5) {
  [0] =>
  string(1) "a"
  [1] =>
  string(1) "y"
  [2] =>
  string(1) "c"
  [3] =>
  string(1) "d"
  [4] =>
  string(1) "e"
}
php > $foo[5] = 'x';
php > var_dump($foo);
array(6) {
  [0] =>
  string(1) "a"
  [1] =>
  string(1) "y"
  [2] =>
  string(1) "c"
  [3] =>
  string(1) "d"
  [4] =>
  string(1) "e"
  [5] =>
  string(1) "x"
}
php > $foo[3] = 'u';
php > var_dump($foo);
array(6) {
  [0] =>
  string(1) "a"
  [1] =>
  string(1) "y"
  [2] =>
  string(1) "c"
  [3] =>
  string(1) "u"
  [4] =>
  string(1) "e"
  [5] =>
  string(1) "x"
}