unset() werkt niet

<?php
$file="gastenboek.htm"; 
$fp=fopen($file, "r"); 
$data=fread($fp, filesize($file)); 
fclose($fp); 
$array = explode("*", $data); 
echo "<br>";

$start=0;
$aantal = count_chars($data); 
echo "aantal berichten  in gastenboek: ".$aantal[ord("*")];  
$eind=$aantal[ord("*")]; 

if(isset($_POST['submit'])=='admin')
{

$file="gastenboek.htm"; 
	$fp=fopen($file, "w"); 
	fputs($fp,unset($array[$_POST['admin']])); //  <<---
	fclose($fp); 
	echo "gelukt";

}
else
{
echo "<form method=post action=admin.php>";

while($start<$eind)
 {
 echo $array[$start]. "<input type=radio name=admin value=$start>";
 $start++;
 }
echo "<input type=submit name=submit value=submit>";
echo "</form>";
}

<?php
$file="gastenboek.htm"; 
$fp=fopen($file, "r"); 
$data=fread($fp, filesize($file)); 
fclose($fp); 
$array = explode("*", $data); 
echo "<br>";
 
$start=0;
$aantal = count_chars($data); 
echo "aantal berichten  in gastenboek: ".$aantal[ord("*")];  
$eind=$aantal[ord("*")]; 
 
if(isset($_POST['submit'])=='admin')
{
 
$file="gastenboek.htm"; 
	$fp=fopen($file, "w"); 
	fputs($fp,unset($array[$_POST['admin']])); //  <<---
	fclose($fp); 
	echo "gelukt";
 
}
else
{
echo "<form method=post action=admin.php>";
 
while($start<$eind)
 {
 echo $array[$start]. "<input type=radio name=admin value=$start>";
 $start++;
 }
echo "<input type=submit name=submit value=submit>";
echo "</form>";
}