Thanks Dave,
Got it worked out.
And instead of trying to write the resulting variable to the db - I write the implode line and it writes the array values to a single record in the database.
Next step to use that to filter a recordset and return only those categories for use as gallery navigation.
<?php do { ?>
<input type="checkbox" name="$galcats[]" value="<?php echo $row_artcategories['Category']; ?>" id="$galcats[]">
<?php echo $row_artcategories['Category']; ?> <br>
<?php } while ($row_artcategories = mysql_fetch_assoc($artcategories)); ?>
<?PHP
if (isset($_POST['$galcats'])) {
$gallerycats = implode(' ',$_POST['$galcats']);
$_SESSION['galcats'] = $gallerycats;
} else {
$gallerycats = '';
}
?>