Community warning: If that string comes from a csv file, use of str_getcsv() instead is strictly advised, as suggested in this answer

Try explode:

$myString = "9,admin@example.com,8";
$myArray = explode(',', $myString);
print_r($myArray);

Output :

Array
(
    [0] => 9
    [1] => admin@example.com
    [2] => 8
)

Use implode:

 $tags = implode(', ', array('tag1','tag2','tag3','tag4'));

$array = explode(',', $variable);

Just like you exploded you can implode again:

$tags = explode(',', $arg);
foreach ($tags as &$tag) {
    $tag = '<a href="' . $u . 'tag/' . $tag . '/" title="' . $tag . '">' . $tag . '</a>';
}

return implode(', ', $tags);

If you need to do this conversion in PHP, as part of whatever import process, then you can just convert the comma-separated values into an array with explode(), and then use update_post_meta(), which will automatically serialise the value for you:

$value = 'Morning,Daytime';
$array = explode( ',', $value );

update_post_meta( $post_id, 'hours_of_operation', $array );

Use trim() if your comma separated values include, or could include, spaces:

$value = 'Morning, Daytime';
$array = explode( ',', $value );
$array = array_map( 'trim', $array );

update_post_meta( $post_id, 'hours_of_operation', $array );

If, for whatever reason, you need to serialise the value yuorself in PHP, you can use serialize() to convert the array to a serialised string.

$value      = 'Morning,Daytime';
$array      = explode( ',', $value );
$serialized = serialize( $array );