Thanks to Brian Fegter. If this answer helps, please rate for Brian's answer right here above.
This is a fully functional example of how to add things to the "header" by its own plugin. In this case, I am adding the properties of Facebook Open Graph for the Share and Like buttons.
Just create a PHP file with the name specified in "Plugin Script" at the beginning of the sample code, place it in a folder with the same name without the extension, obviously, and copy this folder to the destination "/ wp-content / plugins".
Then within "Wordpress", refresh "Plugins" and you'll see your new plugin installed. Just Activate it, and your pages will begin to contain the metadata of Open Graph Facebook and Twitter.
VERY IMPORTANT: The PHP file must be encoded in UTF-8 without BOM, and should have absolutely no character at the end. Must ensure this.
<?php
/*
Plugin Name: My Facebook Open Graph Protocol
Plugin Script: my-facebook-open-graph-protocol.php
Plugin URI:
Description: Add Facebook Open Graph Protocol to header
Author: Diego Soto (Thanks to Brian Fegter)
Donate Link:
License: GPL
Version: 0.1-alpha
Author URI: https://wordpress.stackexchange.com/questions/43672/how-to-add-code-to-header-php-in-a-child-theme
Text Domain: myfogp
Domain Path: languages/
*/
/* Copyright 2014 Diego Soto (http://disientoconusted.blogspot.com.ar/)
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License, version 2, as
published by the Free Software Foundation.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*/
add_action('wp_head', 'wpse_43672_wp_head');
function wpse_43672_wp_head(){
$title = get_the_title() ." ‹ ". get_bloginfo( "name", "display" );
$src = wp_get_attachment_image_src( get_post_thumbnail_id(get_the_ID()), array( 90,55 ), false, "" );
$face_metad = get_post_meta(get_the_ID(), "metadescription", true);
$twitter_metad = get_post_meta(get_the_ID(), "metadescription140", true);
if (empty($twitter_metad))
$twitter_metad = $face_metad;
//Close PHP tags
?>
<meta property="og:title" content="<?php echo esc_attr($title); ?>" />
<meta property="og:image" content="<?php echo esc_attr($src[0]); ?>" />
<meta property="og:url" content="<?php the_permalink(); ?>" />
<meta property="og:description" content="<?php if (!empty($face_metad)) echo esc_attr($face_metad); else the_excerpt(); ?>" />
<meta name="twitter:title" content="<?php echo esc_attr($title); ?>" />
<meta name="twitter:image" content="<?php echo esc_attr($src[0]); ?>" />
<meta name="twitter:url" content="<?php the_permalink(); ?>" />
<meta name="twitter:description" content="<?php if (!empty($twitter_metad)) echo esc_attr($twitter_metad); else the_excerpt(); ?>" />
<?php //Open PHP tags
}
?>
Anyone who is interested in the functionality of the plugin.
The title will be the concatenation of the name of the current page
and the site name.
If a custom field called "metadescription" exists, the plugin tries
to take the description from this field. Otherwise, take the
description from the excerpt.
As the image, the plugin tries to use the thumbnail of the featured
image on the page.