sprintf()'s built-in hex-to-decimal conversion with the %x format character

 
function build_color($red, $green, $blue) {
    return sprintf('#%02x%02x%02x', $red, $green, $blue);
}

1.	Using a sprintf()-style message catalog
2.	Using sprintf with a variable
3.	Using sprintf() to ensure that one digit hex numbers (like 0) get padded with a leading 0.
4.	sprintf print values to a string
5.	Integral that the value be displayed as a dollar amount set to two decimal places:
6.	integer displayed as a dollar amount set to two decimal places
7.	Storing a Formatted String
8.	Wrap these in a couple of functions and surround the joined array elements with parentheses

sprintf()'s built-in hex-to-decimal conversion with the %x format character : sprintf « String « PHP