<table border="5" align="center">
<?php
$a = explode(' ','a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a');
foreach($a as $i => $b){
echo $i % 2 == '0' ? '<tr><td>'.$a[$i].'</td>' : '<td>'.$a[$i].'</td></tr>'."\n";
}
?>
</table>