decimal number with ceil or floor in php

I have a result of decimal number

0.666667

and i wold like to fool it or round it to 0.6,

number_format((float)0.66667, 1, '.', '')
  

but I get 0.7

floor(0.66667)

gives me 0

how can i take this result and floor it to 0.6 or ceil it to 0.7