add a body class based on a post's custom field value

custom-field-advanced-body-class.php

<?php

// check for '_my_custom_field' meta key on pages and page parents and add a body class if meta value equals 'some-value'
	
add_filter('body_class','krogs_custom_field_body_class');

function krogs_custom_field_body_class( $classes ) {

	global $post;

	if ( is_page() && ( 'some-value' == get_post_meta( $post->ID, '_my_custom_field', true ) || 'some-value' == get_post_meta( $post->post_parent, '_my_custom_field', true ) ) ) {
		
		$classes[] = 'some-new-body-class';
		
	}
	
	// return the $classes array
	return $classes;

}

?>

custom-field-body-class.php

<?php

// check for a certain meta key on the current post and add a body class if meta value exists
	
add_filter('body_class','krogs_custom_field_body_class');

function krogs_custom_field_body_class( $classes ) {

	if ( get_post_meta( get_the_ID(), '_my_custom_field', true ) ) {
		
		$classes[] = 'some-new-body-class';
		
	}
	
	// return the $classes array
	return $classes;

}

?>