[SOLVED] How to convert RGBA color codes to 8 Digit Hex in PHP?

How to convert RGBA color codes to 8 Digit Hex in PHP?

I'm attempting to convert some rgba values into a format suitable for SubStation Alpha subtitle files. The .ass file format requires a colour format like &H12345690 where the hex bytes are in blue, green, red, alpha order.

I am finding examples converting 8 digit hex colors into RGBA, but not the other way around. Here is a function I put together based one of the answers, but the alpha channel is always returned as zero:

function rgbtohex($string) {
    $string = str_replace("rgba","",$string);
    $string = str_replace("rgb","",$string);
    $string = str_replace("(","",$string);
    $string = str_replace(")","",$string);
    $colorexplode = explode(",",$string);    
    $hex = '&H';
    
    foreach($colorexplode AS $c) {
        echo "C" . $c . " " . dechex($c) . "<br><br>";
        $hex .= dechex($c);      
    }
    return $hex;
}

But if I test it with rgba(123,223,215,.9) it produces &H7bdfd70 which only has 7 characters instead of 8.

Also, the alpha channel (.9) always comes out to zero, so that doesn't appear to be working correctly.

Solution

You can use the printf() family of functions to convert to a properly padded hex string. Decimals cannot be represented in hex, so the value is taken as a fraction of 0xFF.

$rgba = "rgba(123,100,23,.5)";

// get the values
preg_match_all("/([\\d.]+)/", $rgba, $matches);

// output
$hex = sprintf(
    "&H%02X%02X%02X%02X",
    $matches[1][2], // blue
    $matches[1][1], // green
    $matches[1][0], // red
    $matches[1][3] * 255, // adjusted opacity
);
echo $hex;

Output:

&H17647B7F