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INF 270: WordPress — Week 9: PHP Logic for WordPress

Announcement

No in-person class next Thursday (3/26)!

Keep up with your work on Blackboard (readings and discussion board posts)

Recap: Why PHP Matters in WordPress

Last week we covered PHP syntax and variables. This week we focus on three core skills used constantly in WordPress themes and plugins: functions, conditionals, and loops.

  • Functions let you reuse logic
  • if/else controls what content users see
  • Loops output repeating content like posts and menus

Test your code at https://php-play.dev/

1. Functions in PHP

A function is a reusable block of code. It can accept input (parameters) and optionally send a value back with return. 

<?php
function format_price($amount) {
	return "$" . number_format($amount, 2);
}

$price = format_price(19.99);
echo $price; // $19.99
Built-in functions
Note the example above uses number_format(), a built-in PHP function. You can see the instructions here.

In WordPress, functions are often connected to hooks. Use add_action() to run your function at a specific point in the page lifecycle, and use add_filter() to modify a value before WordPress outputs it. 

<?php
function student_modify_title($title) {
    return "⭐ Featured: " . $title;
}

add_filter('the_title', 'student_modify_title');

Note that this code won't work outside of a WordPress environment. add_filter() and add_action() are specific to WordPress.

The above code can also be written more simply using an anonymous function (closure): 

<?php
add_filter('the_title', function ($title) {
    return "⭐ Featured: " . $title;
});

2. Conditional Logic with if/else

Conditionals allow your page to respond to different situations, such as user login state, post type, category, or custom field values. 

<?php
$score = 78;

if ($score >= 90) {
	echo "A";
} elseif ($score >= 80) {
	echo "B";
} elseif ($score >= 70) {
	echo "C";
} else {
	echo "Needs more work";
}

WordPress has many helper functions designed for conditionals, such as is_user_logged_in(), is_front_page(), and is_single(). 

<?php if (is_user_logged_in()) : ?>
	<p>Welcome back! Here are your account links.</p>
<?php else : ?>
	<p>Please <a href="/my-account">log in</a> to continue.</p>
<?php endif; ?>
Note there are two different ways to "open" and "close" if/then blocks. You can use curly brackets or the alternative syntax with a colon and endif;. 
<?php
// Option 1: Curly brackets
if (is_user_logged_in()) {
	echo "Welcome back!";
} else {
	echo "Please log in.";
}

// Option 2: Alternative syntax
if (is_user_logged_in()) :
	echo "Welcome back!";
else :
	echo "Please log in.";
endif;

Using switch() for multiple exact matches

Use switch() when one variable can match several known options. It is often cleaner than a long chain of if / elseif checks. 

<?php
$template = "archive";

switch ($template) {
	case "front-page":
		echo "Load homepage hero and featured sections.";
		break;
	case "single":
		echo "Load full post layout with author box.";
		break;
	case "archive":
		echo "Load post grid and pagination.";
		break;
	default:
		echo "Load default fallback layout.";
}

3. Loops in PHP

Use loops when you want to repeat output without writing the same markup over and over.

for loop

In a for() loop, you define a starting point, an ending point, and an increment. 

<?php
//this will print out 5 times
//$i starts at 1, and as long as it's less than or equal to 5, 
//it will run the code inside the loop, then increase $i by 1 each time

for ($i = 1; $i <= 5; $i++) {
	echo "Lesson " . $i . "<br />";
}

foreach loop

When working with arrays, a foreach() loop is easier to write and read. It moves through each item in the array and makes it available as a variable. 

<?php
$categories = ["News", "Tutorials", "Announcements"];

foreach ($categories as $category) {
	echo "<li>" . $category . "</li>";
}

while loop

A while() loop keeps running as long as a condition is true. You must update a value inside the loop, or it can run forever. 

<?php
$draft_count = 3;

while ($draft_count > 0) {
	echo "You still have " . $draft_count . " draft posts.<br />";
	$draft_count--;
}

echo "All drafts are published!";

The WordPress Loop

The most important loop in WordPress is the post loop. It checks whether posts exist, then moves through each post and prints content. 

<?php if (have_posts()) : ?>
	<?php while (have_posts()) : the_post(); ?>
		<article>
			<h2><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
			<p><?php the_excerpt(); ?></p>
		</article>
	<?php endwhile; ?>
<?php else : ?>
	<p>No posts found.</p>
<?php endif; ?>

When you work on index.php, archive.php, or single.php templates, this pattern appears constantly.

Lab #5

Head over to Blackboard and start working on Lab #5. You will be creating a custom plugin that uses these concepts to display a dynamic message to customers depending on the value of the items in their cart.