PHP - Reemplazar "strings" diferentes (múltiples)

$text = " Los patitos #nadan* en el lago.";

$text = str_replace("#", "---", $text);

$text = str_replace("*", "---", $text);

echo $text;

<?php

$text = ("Los patos #nadan* en el agua.");

$x = 0;

$treffer1 = strtok ($text, '#');

echo "Nro. 1:".$treffer1;

echo "<br>";

while ( $treffer1 )

{

  $x++;

  $treffer1 = strpos($text, "#",1);

  $treffer1 = strtok('* ');

  $treffer2=str_replace($treffer1,"__________",$text);

  echo '<br>';

 echo $treffer2;

}

?>

<?php

$text = ("Los patos #nadan* en el agua.");

$pos1=strpos ($text,'#');

$pos2=strpos ($text,'*');

$sub=substr($text, $pos1,$pos2-$pos1+1);

echo $text;

echo "<br>";

echo "pos1: ".$pos1;

echo "<br>";

echo "pos2: ".$pos2;

echo "<br>";

echo "sub: ".$sub;

echo "<br>";

$cadena=substr_replace($text,'_________',$pos1,$pos2-$pos1+1);

echo "cadena: ".$cadena;

echo "<br>";

?>

<?php

$string = 'Los patos #nadan* en el agua.';

$pattern = '/(\#(.*)\*)/gi';

$replacement = '________';

echo preg_replace($pattern, $replacement, $string);

?>