Android GCM heads-up notifications not working

I’m building an app for Android using ionic, with a PHP backend.

I want the messages sent to my app to show as heads-up. From what I understand of the documentation, this should be done by setting the priority to 10.

I have successfully sent a message using this code: - however, setting the ‘priority’ attribute there appears to have no effect.

$msg = array
‘message’ => ‘here is a message. message’,
‘title’ => ‘This is a title. title’,
‘subtitle’ => ‘This is a subtitle. subtitle’,
‘tickerText’ => ‘Ticker text here…Ticker text here…Ticker text here’,
‘vibrate’ => 1,
‘sound’ => 1,
‘priority’ => ‘10’
I’ve tried ‘MAX’, ‘HIGH’, 10, and ‘10’ - each time the message is received fine, but does not display as heads-up.

I’m testing on a Google Nexus 5 running Android 6.0.