Check if custom field is populated and echo it or return its value, if any.

wp-custom-meta.php

<?php
/**
 * Custom meta
 *
 * Check if the field is populated and echo it 
 * or return its value, if any.
 *
 * @link https://developer.wordpress.org/reference/functions/get_post_meta/
 *
 * @param  string  $custom_field Unique id for custom field
 * @param  boolean $echo         Whether to echo or return the value
 * @param  string  $before       Custom content before the value
 * @param  string  $after        Custom content after the value
 * @return string                Returns or echoes value for the custom field
 */
function zzap_custom_meta( $custom_field = '', $echo = true, $before = '', $after = '' ) {
	// Translators: post id, custom field id, return a single value
	if ( get_post_meta( get_the_ID(), $custom_field, true ) ) {
		if ( $echo ) {
			echo $before . get_post_meta( get_the_ID(), $custom_field, true ) . $after;			
		} else {
			return get_post_meta( get_the_ID(), $custom_field, true );
		}
	}
}