How to group all terms children's in custom taxonomy?

Question

I have a loop term of my custom taxonomy here:

<?php 
    $terms = get_terms( array( 
       'taxonomy' => 'customtax',
       'hide_empty' => false
    ) );

    $i = 0;                
    foreach( $terms as $term ) {
       echo "
        html block to display $term;
       ";
    $i++;
    };
?>

My goal is to group all terms of parent taxonomy into the array and display one by one:

CAT PARENT NAME:

• children
• children
• children etc...

CAT PARENT NAME 2:

• children
• children
• children etc...

I hope someone understands that.

Answer 1

Here is one of the possible solutions. There are some other recursive solutions for multi child levels. According to your question which is a one child level.

The quick test proved to work which is conducted with default themes and default taxonomy Category.

// first get the parent only $terms
// because of unknown sorting
$parent_terms = [];
$terms = get_terms( array( 
    'taxonomy' => 'category', // change to your taxonomy name
    'hide_empty' => false,
    ) );

foreach( $terms as $term ) {
if( $term->parent == 0 ) {
    $parent_terms[] = $term;
}
};

// create level render
foreach( $parent_terms as $parent ) {
    $child_terms = get_terms( array( 
        'taxonomy' => 'category',
        'child_of' => $parent->term_id,
        'hide_empty' => false,
        ) );

        echo '<h4>' . $parent->name . '</h4>';
        echo '<ul>';
        foreach( $child_terms as $child ) {
        echo '<li>' . $child->name . '</li>';
        }
        unset($child_terms); // avoid garbage for reuse
        echo '</ul>';
}