class-wp-metatags.php

<?php

class WP_Metatags {

	private static $tags = array();

	public static function set_tag( $handle, $content, $type = 'name' ) {
		if ( ! is_array( self::$tags ) ) {
			return new WP_Error( 'metatags_already_printed', __( 'The HTML `meta` tags have already been printed.' ) );
		}

		self::$tags[ $handle ] = array(
			'content' => $content,
			'type'    => $type,
		);
	}

	public static function print_tags() {
		if ( ! is_array( self::$tags ) ) {
			return new WP_Error( 'metatags_already_printed', __( 'The HTML `meta` tags have already been printed.' ) );
		}

		ksort( self::$tags );
		foreach ( self::$tags as $handle => $attributes ) {
			echo self::render_tag( $handle, $attributes ) . "\r\n";
		}
		self::$tags = null;
	}

	public static function render_tag( $handle, $attributes ) {
		switch ( $attributes['type'] ) {
			case 'name' :
				$tag = '<meta name="' . esc_attr( $handle ) . '" ';
				break;
			case 'charset' :
				$tag = '<meta charset="' . esc_attr( $attributes['content'] ) . '" ';
				unset( $attributes['content'] );
				break;
			case 'http-equiv' :
				$tag = '<meta http-equiv="' . esc_attr( $handle ) . '" ';
				break;
			case 'property' : // OG tags use a `property` attribute, not a `name` attribute.
				$tag = '<meta property="' . esc_attr( $handle ) . '" ';
				break;
			default :
				// Uncertain if we should support arbitrary types, or if we should just ignore unrecognized types?
				$tag = '<meta ' . preg_replace( '/[^\da-z\-_]+/i', '', $attributes['type'] ) . '="' . esc_attr( $handle ) . '" ';
				break;
		}

		if ( isset( $attributes['content'] ) ) {
			$tag .= 'content="' . esc_attr( $attributes['content'] ) . '" ';
		}

		$tag .= "/>";

		return $tag;
	}

}

add_action( 'wp_head', array( 'WP_Metatags', 'print_tags' ) );

WP_Metatags::set_tag( 'title',    'Meta Title!' );
WP_Metatags::set_tag( 'charset',  'utf-8',       'charset' );
WP_Metatags::set_tag( 'og:title', 'Overridden!', 'property' );
WP_Metatags::set_tag( 'og:title', 'OG Title!',   'property' );
WP_Metatags::set_tag( 'refresh',  '30',          'http-equiv' );
WP_Metatags::set_tag( 'random',   'value',       'data-blah');

/* Output in wp_head is:

<meta charset="utf-8" />
<meta property="og:title" content="OG Title!" />
<meta data-blah="random" content="value" />
<meta http-equiv="refresh" content="30" />
<meta name="title" content="Meta Title!" />

*/