Explode Function codeigniter not work - php

Question

I'll just try to use explode function in tag system but its not work properly, first half is work but second half is not work, I'll explain my code and issue
Database structure : In a database structure create a one col for tag storage
Tag Col : first_tag,second_tag,tag,third_tag
Now My Code is:
<?php
if(!empty($data)) {
$str1=(explode(",",$data->tags));
$total=count($str1);}
if($data) {
for($i=0;$i<$total;$i++)
{ ?>
<?php echo explode('_',$str1[$i]);?>
<?php }} ?>
Result is : First Explode function is Work Properly
first_tag
second_tag
tag
third_tag
But Second Explode Function is not work : I need This Structure
first tag
second tag
tag
third tag
This Structure I need, please check my code

Answer 1

Maybe something such
if(!empty($data)) {
$str1=(explode(",",$s));
foreach($str1 as $str)
{ ?>
<?php echo str_replace('_', ' ', $str);?>
<?php }
} ?>

Answer 2

use preg_match instead like this
<?php
$data= "first_tag,second_tag,tag,third_tag";
if(!empty($data)) {
$str1=(explode(",",$data));
$total=count($str1);}
if($data) {
for($i=0;$i<$total;$i++)
{
preg_match( "/([a-z]+)_([a-z]+)|([a-z]+)/",$str1[$i],$matches);
?>
<?php echo $matches[1]." ".$matches[2]." ".$matches[3] ;?>
<?php }} ?>
this will produce
first tag
second tag
tag
third tag