PHP echo Hex Numeric solution

PHP echo Hex Value

 
 Echo 0x9f;
?>

Run Program output 159.
159 of the 16 binary is 9f, why the front to add 0x?
And why is it possible to output a 16 binary value directly instead of an error?

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------Solution--------------------
All of the sources derived from C are so, and if you're going to find out why, then you're going to call Bell Labs.

Hexadecimal (hex number system) does not take H (but basic is to take H's) instead to take x, it can be seen that foreigners are not very "rules"
------Solution--------------------
This is C's fault, let you puzzled, accept it

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