explode:

<?php
$tiere = "Hund, Katze, Vogel, Schlange";
$a_tiere = explode(", ",$tiere);
print("<table border='1'><tr><td>");
print(implode("</td></tr><tr><td>",$a_tiere));
print("</td></tr></table>");
?>

oder:

<?php
$tiere = "Hund, Katze, Vogel, Schlange";
$a_tiere = explode(", ",$tiere);
print("<table border='1'>");
foreach($a_tiere as $wert){
	print("<tr><td>".$wert."</td></tr>");
}
print("</td></tr></table>");
?>

Ausgabe

Hund
Katze
Vogel
Schlange