PHP in Templates and Template Conditionals

[DjPrison]

<if ($mybb->user['pms_unread'] >0) then>
<style type="text/css">
.pm_alert 
{
 background: #fff;
}
</style>
<else>
<style type="text/css">
.pm_alert
{
 background: rgba(12,146,0,0.5);
}
</style>
</if>

I have this code but it doesnt work on Mybb 1.8.
I did download the PHP plugin and activate it from ACP.
Im guessing problems are these ">" at "->user" and ">0) then"