It's an undocumented(?) build in Easter Egg.
An Easter egg is an intentional inside joke, hidden message, or feature in an interactive work such as a computer program, video game or DVD menu screen.
In the PHP source code you can find those lines:
if (ta && (ta->tm_mon==3) && (ta->tm_mday==1)) {
php_info_print(PHP_EGG_LOGO_DATA_URI "\" alt=\"PHP logo\" /></a>");
} else {
php_info_print(PHP_LOGO_DATA_URI "\" alt=\"PHP logo\" /></a>");
}
On the first day of the fourth month (1th of April) it does "something special" to the logo on the php_info(). ;-)
To avoid those "jokes" configure PHP with expose_php=off.
A nice collection of some historic PHP Easter Eggs logos can be found on http://www.0php.com/php_easter_egg.php
