unset、array_splice を使用して配列内の要素を削除する方法

<?php 
$arr = array(&#39;a&#39;,&#39;b&#39;,&#39;c&#39;,&#39;d&#39;);
unset($arr[1]);
print_r($arr);
?>

Array ( [0] => a [2] => c [3] => d )

<?php 
$arr = array(&#39;a&#39;,&#39;b&#39;,&#39;c&#39;,&#39;d&#39;); 
array_splice($arr,1,1); 
print_r($arr); 
?>

Array ( [0] => a [1] => c [2] => d )

<?php
$arr2 = array(1,3, 5,7,8);
foreach ($arr2 as $key=>$value)
{
  if ($value === 3)
    unset($arr2[$key]);
}
var_dump($arr2);
?>

<?php
  $array = (&#39;a&#39; => "abc", &#39;b&#39; => "bcd",&#39;c&#39; =>"cde",&#39;d&#39; =>"def",&#39;e&#39;=>"");
  array_filter($array);
  echo "<pre class="brush:php;toolbar:false">";
  print_r($array);
?>

Array ( 
     [a] => abc 
     [b] => bcd 
     [c] => cde 
    [d] => def
)