$string = '9,admin@google.com,8';
$array = explode(',', $string);

For more complicated situations, you may need to use preg_split.

Here's an alternative that uses array_map instead of the foreach loop:

global $u; 
function add_html($tag){
    return('<a href="' . $u . 'tag/' . $tag . '/" title="' . $tag . '">' . $tag .  '</a>');
}
function get_tags($taglist){
    $tags_arr = array_map("add_html", explode( ',' , $taglist));
    return implode(", " , $tags_arr);
} 

use explode function. this will solve your problem. structure of explode is like this

array explode ( string $delimiter , string $string [, int $limit ] )

now $delimiter is the boundary string, string $string is the input string. for limit:

If limit is set and positive, the returned array will contain a maximum of limit elements with the last element containing the rest of string.

If the limit parameter is negative, all components except the last -limit are returned.

If the limit parameter is zero, then this is treated as 1.

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Yes you can do this by using implode

$string = implode(', ', $tags);

And just so you know, there is an alias of implode, called join

$string = join(', ', $tags);

I tend to use join more than implode as it has a better name (a more self-explanatory name :D )