PHP で unset と array_splice の違いを区別する方法

unset ( mixed $var , mixed $... = ? ) : void

  array_splice(array,start,length,array)

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

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