How to convert a string color to its hex code or RGB value?

There is no way to my knowledge to convert HTML color names or CSS color names to their Hex or RGB value in PHP. You'd have to create a map that knows the names (see Wiki article).

Someone has done this work for you already:

• http://psoug.org/snippet/CSS_Colornames_to_RGB_values_415.htm

Excerpt:

$Colors  =  ARRAY( 
    "black"=>array( "red"=>0x00,  "green"=>0x00,  "blue"=>0x00), 
    "maroon"=>array( "red"=>0x80,  "green"=>0x00,  "blue"=>0x00), 
    // ...
    // more colors inbetween
    // ...
    "wheat"=>array( "red"=>0xF5,  "green"=>0xDE,  "blue"=>0xB3), 
    "whitesmoke"=>array( "red"=>0xF5,  "green"=>0xF5,  "blue"=>0xF5), 
    "yellowgreen"=>array( "red"=>0x9A,  "green"=>0xCD,  "blue"=>0x32)
); 

and then simply get the RGB values via $maroon = $Colors['maroon'];

Further reference:

• http://www.w3.org/TR/css3-color/#html4
• http://www.crockford.com/wrrrld/color.html

// converts an html color name to a hex color value
// if the input is not a color name, the original value is returned
// http://wpCodeSnippets.info

function color_name_to_hex($color_name)
{
    // standard 147 HTML color names
    $colors  =  array(
        'aliceblue'=>'F0F8FF',
        'antiquewhite'=>'FAEBD7',
        'aqua'=>'00FFFF',
        'aquamarine'=>'7FFFD4',
        'azure'=>'F0FFFF',
        'beige'=>'F5F5DC',
        'bisque'=>'FFE4C4',
        'black'=>'000000',
        'blanchedalmond '=>'FFEBCD',
        'blue'=>'0000FF',
        'blueviolet'=>'8A2BE2',
        'brown'=>'A52A2A',
        'burlywood'=>'DEB887',
        'cadetblue'=>'5F9EA0',
        'chartreuse'=>'7FFF00',
        'chocolate'=>'D2691E',
        'coral'=>'FF7F50',
        'cornflowerblue'=>'6495ED',
        'cornsilk'=>'FFF8DC',
        'crimson'=>'DC143C',
        'cyan'=>'00FFFF',
        'darkblue'=>'00008B',
        'darkcyan'=>'008B8B',
        'darkgoldenrod'=>'B8860B',
        'darkgray'=>'A9A9A9',
        'darkgreen'=>'006400',
        'darkgrey'=>'A9A9A9',
        'darkkhaki'=>'BDB76B',
        'darkmagenta'=>'8B008B',
        'darkolivegreen'=>'556B2F',
        'darkorange'=>'FF8C00',
        'darkorchid'=>'9932CC',
        'darkred'=>'8B0000',
        'darksalmon'=>'E9967A',
        'darkseagreen'=>'8FBC8F',
        'darkslateblue'=>'483D8B',
        'darkslategray'=>'2F4F4F',
        'darkslategrey'=>'2F4F4F',
        'darkturquoise'=>'00CED1',
        'darkviolet'=>'9400D3',
        'deeppink'=>'FF1493',
        'deepskyblue'=>'00BFFF',
        'dimgray'=>'696969',
        'dimgrey'=>'696969',
        'dodgerblue'=>'1E90FF',
        'firebrick'=>'B22222',
        'floralwhite'=>'FFFAF0',
        'forestgreen'=>'228B22',
        'fuchsia'=>'FF00FF',
        'gainsboro'=>'DCDCDC',
        'ghostwhite'=>'F8F8FF',
        'gold'=>'FFD700',
        'goldenrod'=>'DAA520',
        'gray'=>'808080',
        'green'=>'008000',
        'greenyellow'=>'ADFF2F',
        'grey'=>'808080',
        'honeydew'=>'F0FFF0',
        'hotpink'=>'FF69B4',
        'indianred'=>'CD5C5C',
        'indigo'=>'4B0082',
        'ivory'=>'FFFFF0',
        'khaki'=>'F0E68C',
        'lavender'=>'E6E6FA',
        'lavenderblush'=>'FFF0F5',
        'lawngreen'=>'7CFC00',
        'lemonchiffon'=>'FFFACD',
        'lightblue'=>'ADD8E6',
        'lightcoral'=>'F08080',
        'lightcyan'=>'E0FFFF',
        'lightgoldenrodyellow'=>'FAFAD2',
        'lightgray'=>'D3D3D3',
        'lightgreen'=>'90EE90',
        'lightgrey'=>'D3D3D3',
        'lightpink'=>'FFB6C1',
        'lightsalmon'=>'FFA07A',
        'lightseagreen'=>'20B2AA',
        'lightskyblue'=>'87CEFA',
        'lightslategray'=>'778899',
        'lightslategrey'=>'778899',
        'lightsteelblue'=>'B0C4DE',
        'lightyellow'=>'FFFFE0',
        'lime'=>'00FF00',
        'limegreen'=>'32CD32',
        'linen'=>'FAF0E6',
        'magenta'=>'FF00FF',
        'maroon'=>'800000',
        'mediumaquamarine'=>'66CDAA',
        'mediumblue'=>'0000CD',
        'mediumorchid'=>'BA55D3',
        'mediumpurple'=>'9370D0',
        'mediumseagreen'=>'3CB371',
        'mediumslateblue'=>'7B68EE',
        'mediumspringgreen'=>'00FA9A',
        'mediumturquoise'=>'48D1CC',
        'mediumvioletred'=>'C71585',
        'midnightblue'=>'191970',
        'mintcream'=>'F5FFFA',
        'mistyrose'=>'FFE4E1',
        'moccasin'=>'FFE4B5',
        'navajowhite'=>'FFDEAD',
        'navy'=>'000080',
        'oldlace'=>'FDF5E6',
        'olive'=>'808000',
        'olivedrab'=>'6B8E23',
        'orange'=>'FFA500',
        'orangered'=>'FF4500',
        'orchid'=>'DA70D6',
        'palegoldenrod'=>'EEE8AA',
        'palegreen'=>'98FB98',
        'paleturquoise'=>'AFEEEE',
        'palevioletred'=>'DB7093',
        'papayawhip'=>'FFEFD5',
        'peachpuff'=>'FFDAB9',
        'peru'=>'CD853F',
        'pink'=>'FFC0CB',
        'plum'=>'DDA0DD',
        'powderblue'=>'B0E0E6',
        'purple'=>'800080',
        'red'=>'FF0000',
        'rosybrown'=>'BC8F8F',
        'royalblue'=>'4169E1',
        'saddlebrown'=>'8B4513',
        'salmon'=>'FA8072',
        'sandybrown'=>'F4A460',
        'seagreen'=>'2E8B57',
        'seashell'=>'FFF5EE',
        'sienna'=>'A0522D',
        'silver'=>'C0C0C0',
        'skyblue'=>'87CEEB',
        'slateblue'=>'6A5ACD',
        'slategray'=>'708090',
        'slategrey'=>'708090',
        'snow'=>'FFFAFA',
        'springgreen'=>'00FF7F',
        'steelblue'=>'4682B4',
        'tan'=>'D2B48C',
        'teal'=>'008080',
        'thistle'=>'D8BFD8',
        'tomato'=>'FF6347',
        'turquoise'=>'40E0D0',
        'violet'=>'EE82EE',
        'wheat'=>'F5DEB3',
        'white'=>'FFFFFF',
        'whitesmoke'=>'F5F5F5',
        'yellow'=>'FFFF00',
        'yellowgreen'=>'9ACD32');

    $color_name = strtolower($color_name);
    if (isset($colors[$color_name]))
    {
        return ('#' . $colors[$color_name]);
    }
    else
    {
        return ($color_name);
    }
}