I think sparky's answer is good but addresses a slightly different question. Perhaps this is closer to the question as posed:
Code:
<?php
$ckeys = array();
$ckeys['keywords'] = <<<EOT
word1
word2
word3
word4
EOT;
foreach (explode("\n", $ckeys['keywords']) as $i => $w) {
$id = "cell" . ($i+1);
echo "<p id='$id'>$w</p>\n";
}
?>
fg