How filters work in WordPress, aka how to make your plugins and functions extendable and the output easily modifiable. | +Tags: filter, filters, extend

wp-filters.php

<?php

function eyn_wb17_excerpt_from_content( $content, $wordlimit, $url, $readmore = 'Read More' ) {

	// Action hook before any output!
	do_action( 'eyn_wb17_before_excerpt' );

	// Make the Read More text filterable
	$readmore = apply_filters( 'eyn_wb17_excerpt_rmtext', $readmore );

	$rmString = ' ... <a href="' . $url . '">' . $readmore . '</a>';
	// Make the Read More string filterable
	$rmString = apply_filters( 'eyn_wb17_excerpt_rmstring', $rmString );

	$excerpt = force_balance_tags( 
		html_entity_decode( 
			wp_trim_words( 
				htmlentities( 
					wpautop( $content ) 
				), 
				$wordlimit, 
				$rmString 
			) 
		) 
	);

	echo $excerpt; 

	// Action hook after rest of output!
	do_action( 'eyn_wb17_after_excerpt' );

}

// Filter: Replace the readmore text
add_filter( 'eyn_wb17_excerpt_rmtext', 'eyn_wb17_excerpt_rmtext_mod' );
function eyn_wb17_excerpt_rmtext_mod( $text ) {
	return 'Read Me Later';
}

// Filter: Replace the readmore string
add_filter( 'eyn_wb17_excerpt_rmstring', 'eyn_wb17_excerpt_rmstring_mod' );
function eyn_wb17_excerpt_rmstring_mod( $text ) {
	$text = str_replace( 
		array( '...' ),
		array( '>>>' ),
		$text
	);
	return $text;
}

// Action: Add title before excerpt
add_action( 'eyn_wb17_before_excerpt', 'eyn_wb17_excerpt_addbefore' );
function eyn_wb17_excerpt_addbefore() {
	echo '<h4>Beautiful Content Excerpt for ' . date( 'F j, Y') . ':</h4>';
}

// Action: add something after excerpt
add_action( 'eyn_wb17_after_excerpt', 'eyn_wb17_excerpt_addafter' );
function eyn_wb17_excerpt_addafter() {
	echo '<div><img src="http://kacaj.eu/feltoltesek/2011/12/trollface-400x400.jpg" width="150" height="150" alt="Problem?"></div>';
}