function my_tiny_mce_before_init( $mceInit ) {
$style_formats = array(
array(
'title' => 'Code language',
'items' => array(
array(
'title' => 'PHP',
'block' => 'code',
'classes' => 'language-php'
),
array(
'title' => 'CSS',
'block' => 'code',
'classes' => 'language-css'
),
array(
'title' => 'HTML',
'block' => 'code',
'classes' => 'language-html'
)
)
) 
);

$mceInit['style_formats'] = json_encode( $style_formats );

return $mceInit; 
}
add_filter( 'tiny_mce_before_init', 'my_tiny_mce_before_init' );