Here is one way you can randomly change the background color in PHP.
<?php
print "<html>\n";
$intRed = rand(0,255); // generates a random number between 0 and 255
$intGreen = rand(0,255);
$intBlue = rand(0,255);
$hexRed = dechex($intRed+0); // converts the number to hex
$hexGreen = dechex($intGreen+0);
$hexBlue = dechex($intBlue+0);
$strRed=$hexRed;
// makes the value two characters wide so red, green, and blue each take two characters in the RGB value
if (strlen($strRed) == 1) { $strRed="0".$strRed; }
$strGreen=$hexGreen;
if (strlen($strGreen) == 1) { $strGreen="0".$strGreen; }
$strBlue=$hexBlue;
if (strlen($strBlue) == 1) { $strBlue="0".$strBlue; }
$hexRGB=$strRed.$strGreen.$strBlue; // joins the three string vales to make a RGB value
print "<body bgcolor=$hexRGB>\n";
print "The background page color is $hexRGB\n";
print "</body>\n";
print "</html>\n";
?>