How PHP converts an unknown number to an integer

How PHP converts an unknown number to an integer
Beginner PHP, encounter a little problem do not understand, forget you give me a little guidance, thank you!

 
 Echo ((0.1+0.7) *10)              //Display 8
Echo? (int) ((0.1+0.7) *10);?       How to display the display? 7?
Echo Intval (((0.2+0.7) *10);     Take the integer 9, which is correct
Echo Intval (((0.1+0.7) *10);     How to show 7 again?
Echo (Integer) (((0.1+0.7) *10));  Or is it 7?
?>

How is it that the result (integer or floating-point) of an expression similar to ((0.1+0.7) *10) is converted to an integer? PHP

------Solution--------------------
On the accuracy of floating-point numbers

$n = ((0.1+0.7) *10);
printf ("%.16f%d\n", $n, $n);

7.9999999999999991 7

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