If I give phpthumbof an image with size of 78x78 and give these parameters: w=283&h=248&f=png&zc=1 it returns pixelated upresized image.
Source image: http://srv39820.ht-test.ru/28-5.icon-vk.png
Result image: http://srv39820.ht-test.ru/28-5.icon-vk_283x248_3c1.png
This problem came up after moving to other hosting. On previous one the result image was equal to source image.
You should check image size before resizing.
now if width and height are smaller then required, don't resize and just save your image.