<?php
$string = '<img alt="" src="{{url}}" style="height:266px; width:400px">';
echo preg_replace("/style.*(height):(\d+).*(width):(\d+).*\"/ui", '$1="$2" $3="$4"', $string);
PD9waHAKCiRzdHJpbmcgPSAnPGltZyBhbHQ9IiIgc3JjPSJ7e3VybH19IiBzdHlsZT0iaGVpZ2h0OjI2NnB4OyB3aWR0aDo0MDBweCI+JzsKCmVjaG8gcHJlZ19yZXBsYWNlKCIvc3R5bGUuKihoZWlnaHQpOihcZCspLiood2lkdGgpOihcZCspLipcIi91aSIsICckMT0iJDIiICQzPSIkNCInLCAkc3RyaW5nKTs=