What does "0x" mean in hexadecimal numbers?

It has already been answered by Lucas that "0x prefix identifies the number that follows as a hexadecimal constant", but I want to complement the answer with some relevant details:

  

P: To use a prefix?
A: To differentiate from a decimal, since hexa is perfectly normal with no letter at all. 0x99 , for example, is 153 in decimal, but if you typed int x = 99 , the compiler would not have guessed its intention. the 0x99 already makes explicit that it is hexa.

Curiosities:

This is a real trickster trick: in some languages, 0 (without x ) before numbers, indicates "octal", so x = 032 is the same as x = 26 . This easily confuses those who have no experience with prefixes.

I have already seen certain BASIC dialects using &b or 0b to indicate binary, for example 0b00001011 to represent decimal 11 , as in some cases &h to hexadecimal (in MSX , &h8000 is the usual way to represent the beginning of available memory for writing after a normal boot.)

One of the languages I use a lot (Harbor), uses 0d for dates and 0t for timestamp. For example, dNascimento := 0d20010527 .

0x or 0X is a prefix that was initially used by AT & T assembly compilers in the late 1960s to represent hexadecimal numeric values.

Bell Laboratories , at the time an AT & T subsidiary, was the first to adopt the standard. It is also known for creating the UNIX operating environment, where it made extensive use of this notation. Several * NIX syntactic descendants (C, C #, Java, JavaScript, and others) have propagated usage to this day.

The initial% (zero)% indicates that the value is a numeric constant; 0 is phonetically similar in English, with 'Hexa'.

The prefix 0x identifies the number that follows as a hexadecimal constant, and the prefix 0 as an octal number (base 8).

For example,

#include <iostream>
using namespace std;

int main() {
    cout << 0xF << endl;
    cout << 010 << endl;
    return 0;
}

It will print 16 and 8 on the screen.

Some languages define 0x as a prefix of a hexadecimal number, basically it is a signaling to the compiler / interpreter that the number should be treated in another base (16).

whole php