Example/pages/foo.php
<?php
echo '<p>Here is a link to <a href="', plugin_page( 'foo' ), '">page foo</a>.</p>';
     '<link rel="stylesheet" type="text/css" href="', plugin_file( 'foo.css' ), '"/>',
     '<p class="foo">';

event_signal( 'EVENT_EXAMPLE_FOO' );

$t_string = 'A sentence with the word "foo" in it.';
$t_new_string = event_signal( 'EVENT_EXAMPLE_BAR', array( $t_string ) );

echo $t_new_string, '</p>';