转换为3位十六进制颜色代码

I've been using 3-digit hex color values in CSS for a long time: #fff, #999, #069, etc. I can see how the repeating letters/numbers are merged to create a 3-digit hex color code, but I don't fully understand the pattern to be able to write a converter in PHP. Is there documentation for this?

Edit: Oh, perhaps my question wasn't clear. I need to know how some of the 6-digit hex color values are converted to 3-digits. xxxxxx (ffffff) and xxyyzz (006699) – these are the only two patterns, correct?

To convert a 3-character hex code into a 6 character one, you need to repeat each character:

$hex = '#fff';
$hex6 = '#' . $hex[1] . $hex[1] . $hex[2] . $hex[2] . $hex[3] . $hex[3];

If you want to convert it to decimal you can use the hexdec function

3 digit CSS code is short for 6 digits": #06a; is #0066aa;
Each two digits represent a number from 0 to 255.
Converting these values to hex and back is all you need.

#f0f is expanded to #ff00ff so basically you just need to calculate the value and the value times 16 for each character, e.g.:

#f98: f = 15 => red = 15 + 15*16 = 255 etc.

function hexfix(str) {
  var v, w;
  v = parseInt(str, 16);    // in rrggbb
  if (str.length == 3) {
    // nybble colors - fix to hex colors
    //  0x00000rgb              -> 0x000r0g0b
    //  0x000r0g0b | 0x00r0g0b0 -> 0x00rrggbb
    w = ((v & 0xF00) << 8) | ((v & 0x0F0) << 4) | (v & 0x00F);
    v = w | (w << 4);
  }
  return v.toString(16).toUpperCase();
 }

var hex1 = 'AABBCC',
    hex2 = 'ABC';

document.body.appendChild(document.createTextNode(hex1+" becomes "+hexfix(hex1)+'.  '));
document.body.appendChild(document.createTextNode(hex2+" becomes "+hexfix(hex2)+'.  '));

Something like this.

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