setup_postdata() │ WP 1.5.0

This example is taken from a user comment:

Question:

Greetings! I think I'm missing something:

1) Such a code:

$top_posts = get_posts([
	'posts_per_page' => 5,
]);

if ( $top_posts ) {
	foreach ( $top_posts as $post ) {
		// setup_postdata($post);
		the_title();
		echo '<hr>';
	}
}
wp_reset_postdata();

It works fine, it outputs what you need. Why use setup_postdata($post);?

2) Such a code:

$top_posts = get_posts([
	'posts_per_page' => 5,
]);

if ( $top_posts ) {
	foreach ($top_posts as $item) {
		setup_postdata($item);
		the_title();
		echo '<hr>';
	}
}
wp_reset_postdata();

Outputs 5 times the title of the first post. Here I don't understand why at all? Is the name of $post variable mandatory when looping?

Answer:

1 code

In the description setup_postdata(), it says what it is for:

Fills global variables: $id, $authordata, $currentday, $currentmonth, $page, $pages, $multipage, $more, $numpages. They are needed for the correct operation of the Template Tags, which must be used within the WordPress Loop.

I.e. without it some tags of the template will not work properly! For the_title() it is not needed...

2 code

the_title() and similar ones work exactly with $post, by specifying $post for foreach you sort of change the global $post value to the current, but if you specify $item, no override occurs, so 5 identical headers...

setup_postdata() does not appear in this question, it has another task: to define other global variables besides $post needed for the template tags. For example, you could write it like this:

...
	foreach ($top_posts as $item) {
		$post = $item; // rewrite the glob. `$post`
		setup_postdata($item);
		the_title();
		echo '<hr>';
	}
...

And from this code you can see that it's easier to use $post...

Also, you don't have $post defined as global in your code, which means that your code is in some file directly (i.e. it is in the global scope of PHP) and therefore $post is already global there. So for example, if you wrap your code in a function, you would have to write global $post at the beginning of the function so that $post is global when used inside the function (in the local scope of the code). Otherwise template tags like the_title() will not work properly.

Let's say we get the post data into the $post_object variable, then we need to create a variable called $post and pass that data into it. setup_postdata() works only with the global variable $post.

global $post;

// Pass the received post data to the $post variable and not to any other.
$post = $post_object;

setup_postdata( $post );
// the_title();
// the_permalink();

A short record of the example above:

setup_postdata( $GLOBALS['post'] = & $post_object );

And here is a wrong variant:

global $post;

setup_postdata( $post_object ); // bad

Get posts from category 1 and display them as links:

<ul>
	<?php
	global $post;
	$myposts = get_posts( array( 'numberposts' => 5, 'offset'=> 1, 'category' => 1 ) );
	foreach( $myposts as $post ){
		setup_postdata($post);
		?>
		<li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
		<?php 
	}
	wp_reset_postdata(); // important
	?>
</ul>

<ul>
	<?php
	global $wpdb, $post;
	$result = $wpdb->get_results(
		"SELECT * FROM $wpdb->posts WHERE post_type = 'post' AND post_status = 'publish'"
	);
	foreach( $result as $post ){
		setup_postdata( $post );
		?>
		<li><a href="<?php the_permalink()?>"><?php the_title();?></a></li>
		<?php 
	}
	wp_reset_postdata(); // important
	?>
</ul>

Demonstration of what happens if you pass not the global variable $post as an argument, but any other - the function will not work as it expected.

Let's pass $pst, but not $post:

global $post;

$posts = get_posts( "posts_per_page=2" );

foreach( $posts as $pst ){
	setup_postdata( $pst );
	echo the_title();
	echo '<br>';
}

echo '$post->post_title - '. $post->post_title . '<br>';

/*
Display:
current global post title
current global post title
$post->post_title - current global post title
*/

Now lets pass normal $post:

// repeat only with $post instead of $pst
$posts = get_posts( "posts_per_page=2" );

foreach( $posts as $post ){
	setup_postdata( $post );
	echo the_title();
	echo '<br>';
}

echo '$post->post_title - '. $post->post_title . '<br>';

/*
Display:
Received first post title
Received second post title
$post->post_title - Received second post title
*/