add class to body class wordpress

PHP
<body <?php body_class( 'class-name' ); ?>>
add_filter( 'body_class', 'custom_class' );
function custom_class( $classes ) {
    if ( is_page_template( 'page-example.php' ) ) {
        $classes[] = 'example';
    }
    return $classes;
}
<body <?php body_class(); ?>>

Source

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