This function assigns NULL to the array element. Its use has no effect on count(). Whereas assigning NULL to an element will have no effect on isset(), offsetUnset() and unset() do. Whereas unsetting an element affects the behaviour of foreach() as applied to an array or an ArrayObject, it has no such effect on SplFixedArray, as demonstrated by the code below.

<?php

class atest extends SplFixedArray {
  public function fill() {
    for ($i = $this->count(); --$i >= 0; ) $this[$i] = $i;
  }
  public function dump() {
    $sep = ' ';
    foreach ($this as $k => $v) {
      echo $sep, "$k: ", (is_null($v) ? 'NULL' : $v);
      if (!isset($this[$k])) echo ' and unset';
      $sep = ', ';
    }
    echo PHP_EOL;
  }
}

$a = new atest(3);
$a->dump(); //  0: NULL and unset, 1: NULL and unset, 2: NULL and unset
$a->fill();
$a->dump(); //  0: 0, 1: 1, 2: 2
$a[1] = NULL;
unset($a[2]);
$a->dump(); //  0: 0, 1: NULL, 2: NULL and unset
?>