Echoing Categories only if they’re a child of a given category

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I’ve got posts that I want to echo after the post content the categories only if they’re a child of a given parent category.

Example:

Parent category: Goal
Child categories of “Goal”: brand awareness, brand engagement

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If a post is categorized, I want it to echo out those category or categories: 

<?php
    $categories = get_the_category();
    $seperator = ' ';
    $output = '';
    if($categories)
    {
        foreach($categories as $category) 
        {
            $output .= '<a href="'.get_category_link($category->term_id ).'" title="' . esc_attr( sprintf( __( "View all posts in %s" ), $category->name ) ) . '">'.$category->cat_name.'</a>'.$seperator;
        }
        echo trim($output, $seperator);
    }
?>

I’m using this, but it obviously outputs all categories, rather than just the ones that are children of the “goal” slug.

Edit: Current setup looks like this, but doesn’t seem to be working. If I use the ID, it’s fine: 

<?php
        $categories = get_the_category();
        $seperator = ', ';
        $output = '<strong>GOAL:</strong> ';
        $category = get_category_by_slug('goal');
        if($categories)
        {
            foreach( $categories as $category ) 
            {
            if ( $category->parent == $category->term_id )
                $output .= '<a href="'.get_category_link($category->term_id ).'" title="' . esc_attr( sprintf( __( "View all posts in %s" ), $category->name ) ) . '">'.$category->cat_name.'</a>'.$seperator;
        }
        echo trim($output, $seperator);
}
?>
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4 comments

  1. Change:

    $goal = get_category_by_slug('goal');
foreach($categories as $category) 
{
    $output .= '<a href="'.get_category_link($category->term_id ).'" title="' . esc_attr( sprintf( __( "View all posts in %s" ), $category->name ) ) . '">'.$category->cat_name.'</a>'.$seperator;
}

    Into:

    foreach( $categories as $category ) {
    if ( $category->parent == $goal->term_id)
        $output .= '<a href="'.get_category_link($category->term_id ).'" title="' . esc_attr( sprintf( __( "View all posts in %s" ), $category->name ) ) . '">'.$category->cat_name.'</a>'.$seperator;
}

    Where you replace '123' with the ID of the Goal category.

  2. I agree with the above answer except that it should use get_category_by_slug() instead of hardcoded ID’s. The comments got sort of confusing to follow because the question was also changed – so I’ve split this out into my own answer.

    To work this into the bit of logic you have above…

    <?php
    $goal_id = get_category_by_slug('goal');

    $categories = get_the_category();
    $seperator = ' ';
    $output = '';
    if($categories)
    {
        foreach($categories as $category) 
        {
            if($category->parent == $goal->term_id){
                $output .= '<a href="'.get_category_link($category->term_id ).'" title="' . esc_attr( sprintf( __( "View all posts in %s" ), $category->name ) ) . '">'.$category->cat_name.'</a>'.$seperator;
            }
        }
        echo trim($output, $seperator);
    }
?>

    I am assuming that the slug of your “Goal” category is simply goal.

  3. What about using a conditional check?

    here’s example which checks for parent category with ID (4).

    Example –

    //To show child  of cat with id - 4
if($category->category_parent == '4') 
    $output .= 'foo';