What is the meaning of \x00 , \x04 in PHP

i have some codes having the \x00 and \x04 hex codes, what does it means?

$str= implode("\x00", $var['message']); //line 1
$id= $var['message'] . "\x04" . $id;    //line 2

what will happen in the line1 and line2 I want to write these into a external file as binary format.

where do i get all information like this.

\x indicates hexadecimal notation. See: PHP strings

Have a look at an ASCII table to see what 0x00 and 0x04 represent.

0x00 = NULL
0x04 = EOT (End of transmission)

\x is a way to indicate that the next two characters represent hexadecimal digits. Two hexadecimal digits (each of them 4 bits) make a byte.

If you want to know what the decimal version is, multiply the left numeral by 16 and add it to the right numeral, keeping in mind that "a" is 10, "b" is 11, etc.

In other programming languages, a dollar sign or the sequence 0x can also be used to flag hexadecimal numbers.

The numbers can represent anything. Sometimes they are control codes. Check an ASCII table.