An example of how to use WordPress Conditional Tags inside of custom functions.

freelancer-wordpress-conditionals-example.php

add_action( 'theme_before_content', 'my_banner_ad' );
/*
 * This conditional example ensures that the banner ad only
 * displays on the front page by returning nothing if it
 * is NOT the front page.
 */
function my_banner_ad() {

	if ( ! is_front_page() )
		return;
	
	echo '<img src="http://example.com/link-to-my-banner.png">';

}

add_action( 'theme_before_content', 'my_banner_ad' );
/*
 * This conditional example ensures that the banner ad only
 * displays on the front page by only echoing the image code
 * if it IS the front page. (same result as above, but different method)
 */
function my_banner_ad() {

	if ( is_front_page() ) {

		echo '<img src="http://example.com/link-to-my-banner.png">';

	}

}

add_action( 'theme_before_content', 'my_banner_ad' );
/*
 * This conditional example ensures that the banner ad only
 * displays on the front page OR a single page by only echoing the image code
 * if it IS the front page OR it is a single page.
 */
function my_banner_ad() {

	if ( is_front_page() || is_page() ) {

		echo '<img src="http://example.com/link-to-my-banner.png">';

	}

}