There a couple of ways you can go about it depending on your use case and although example number 1 is perfectly valid, it might make more sense to employ the use of example number 2.
Example #1:
This will send you an email each time a post is updated/published with the status of “pending”. This will also email you if the status was already “pending” and the user hits save as pending again.
add_action( 'transition_post_status', 'pending_post_status', 10, 3 );
function pending_post_status( $new_status, $old_status, $post ) {
if ( $new_status === "pending" ) {
wp_mail(
//your arguments here...
);
}
}
Example #2:
This will send you an email when a post is updated or published as pending only IF the previous status of the post was not already set to pending.
add_action( 'transition_post_status', 'pending_post_status', 10, 3 );
function pending_post_status( $new_status, $old_status, $post ) {
if ( $new_status === "pending" && $old_status !== "pending" ) {
wp_mail(
//your arguments here...
);
}
}
Choose your poison…
Additional resources: