str_replace

echo "<pre>";
print_r($gegevens);
echo "</pre>";

echo "<pre>";
print_r($gegevens);
echo "</pre>";

<?php
error_reporting(E_ALL);
if($_SERVER['REQUEST_METHOD'] == "POST"){
    $text = nl2br($_POST['text']);


    echo $text;
    echo "<br /><br />";
    $aantal = strlen($text)-1;
 
$bestand = "top.txt";
$openbestand = fopen($bestand, "r");
$gegevens = fread($openbestand, filesize($bestand));
fclose($openbestand);
$gegevens = explode("\n",$gegevens);
 		
echo "<br /><br />Resultaat:<br />";
echo str_replace($gegevens, "hoi", $text);

}else{
      ?>


<form method="post" action="<?=$_SERVER['PHP_SELF'];?>">
<table><td valign="top">Vul hier uw tekst in: </td>
<td><textarea name="text" cols="40" rows="15"></textarea></td></tr>
<tr><td><input type="submit" value="Verzenden"></td><td>&nbsp;</td>
</table>
</form>
<?php
}
?>

<?php
error_reporting(E_ALL);
if($_SERVER['REQUEST_METHOD'] == "POST"){
    $text = nl2br($_POST['text']);
 
 
    echo $text;
    echo "<br /><br />";
    $aantal = strlen($text)-1;
 
$bestand = "top.txt";
$openbestand = fopen($bestand, "r");
$gegevens = fread($openbestand, filesize($bestand));
fclose($openbestand);
$gegevens = explode("\n",$gegevens);
 
echo "<br /><br />Resultaat:<br />";
echo str_replace($gegevens, "hoi", $text);
 
}else{
      ?>
 
 
<form method="post" action="<?=$_SERVER['PHP_SELF'];?>">
<table><td valign="top">Vul hier uw tekst in: </td>
<td><textarea name="text" cols="40" rows="15"></textarea></td></tr>
<tr><td><input type="submit" value="Verzenden"></td><td>&nbsp;</td>
</table>
</form>
<?php
}
?>