php two arrays into string - php

Question

With php print_r I got final result as :
Array ( [0] => 31 [1] => 21 ) Array ( [0] => 33 [1] => 27 )
And I want result to be like 31,21,33,27.
I am stuck up. Please guide me through.
Thanks,
Vikram
**Please bare with lengthy stuff. **
I have 2 main categories and need to display 3 fourth level children on home page. Categories are like :
Category > Clothing.
Category > Clothing > Mens.
Category >> Clothing > Mens > Top.
Category > Clothing > Mens > Top > Jeans.
CODES
<div class="container products" data-aos="fade-down">
<div class="page-titles" data-aos="fade-up">
Products
<hr />
</div>
<!--page-titles-->
<?php
$homepage_products=get_post_meta($post->ID, 'homepage_products', TRUE);
$homepage_parent=explode(',', $homepage_products);
global $parent_one;
global $x;
$x=1;
foreach($homepage_parent as $parent_one)
{
$parent_one_title = get_the_title( $parent_one );
$content_post = get_post($parent_one);
$content = $content_post->post_content;
$content = apply_filters('the_content', $content);
?>
<div class="row clothing">
<div id="<?php echo $x++; ?>" class="col-xs-12 col-sm-12 col-md-4 col-lg-4
<?php if($x % 2)
{
echo 'col-md-push-8 col-lg-push-8';
}
?>
">
<?php echo get_the_post_thumbnail($parent_one, 'medium_large', array('class' => 'img-'.$x)); ?>
</div>
<!--col-4 img-->
<div class="col-xs-12 col-sm-12 col-md-8 col-lg-8
<?php if($x % 2)
{
echo 'col-md-pull-4 col-lg-pull-4';
}
?>
">
<h2><?php echo $parent_one_title; ?></h2>
<?php echo $content; ?>
<!-- child pages of <?php echo strtolower($parent_title); ?> starts-->
<div class="row">
<?php
$parent_two=get_pages( array('parent'=>$parent_one,'child_of'=>$parent_one) );
foreach($parent_two as $parent_two_ids)
{
$parent_two_id=$parent_two_ids->ID;
$parent_two_id.=",";
$parent_three=get_pages( array('parent'=>$parent_two_id) );
$str_id = array();
foreach($parent_three as $parent_three_ids)
{
$parent_three_id=$parent_three_ids->ID;
$parent_three_id.=',';
$parent_four=get_pages( array('parent'=>$parent_three_id) );
$four_ids=array();
foreach($parent_four as $parent_four_ids)
{
$four_id=$parent_four_ids->ID;
$four_ids[]=$four_id;
}//parent_four_ids
// HERE IS MY ISSUE //
echo implode(',', array_merge($four_ids) );
// HERE IS MY ISSUE //
//$ids=implode(',',$four_ids);
//$idx=$ids;
//$idv=preg_replace('#\s+#',',',trim($idx));
//echo $idv;
//$str_id[]=$idx;
//$str_idx=implode(',',$str_id);
//echo $str_idx;
$content=apply_filters('the_content', $parent_four_ids->post_content);
$c_length = 80;
if (strlen($content) > $c_length)
{
$content = wordwrap($content, 80);
$i = strpos($content, "\n");
if ($i) {
$content = substr($content, 0, $i);
}
}
?>
<!-- <div class="col-xs-12 col-sm-12 col-md-3 col-lg-3 product">
< ?php echo get_the_post_thumbnail($parent_four_ids, 'medium_large'); ?>
<a href="< ?php echo get_permalink($parent_four_ids);?>">
<h4>< ?php echo $parent_four_ids->post_title; ?></h4>
</a>
< ?php echo $content; ?>
</div><!--col 3 #< ?php echo $parent_four_ids->ID; ?>-->
<?php
}//parent_three_ids
} // parent_two_ids
?>
</div>
<!-- row category - slider- child pages of <?php echo strtolower($parent_one_title); ?> ends-->
</div>
<!-- col-6 content - child pages-->
</div>
<!--row <?php echo strtolower($parent_title); ?>-->
<? } //home_parent ?>
</div>
<!--container products-->

Answer 1

Just as iainn said above:
php > echo implode(',', array_merge([31, 21], [33, 27]));
31,21,33,27
If you wanted a single array and not a string, remove the implode() call.