<a href="whatsapp://send?text=Hello%20World!">Hello, world!</a>

NSURL *whatsappURL = [NSURL URLWithString:@"whatsapp://send?text=Hello%2C%20World!"];
if ([[UIApplication sharedApplication] canOpenURL: whatsappURL]) {
    [[UIApplication sharedApplication] openURL: whatsappURL];
}

<?php
$iPod    = stripos($_SERVER['HTTP_USER_AGENT'],"iPod");
$iPhone  = stripos($_SERVER['HTTP_USER_AGENT'],"iPhone");
$iPad    = stripos($_SERVER['HTTP_USER_AGENT'],"iPad");
$Android = stripos($_SERVER['HTTP_USER_AGENT'],"Android");
if( $iPod || $iPhone || $iPad ) {
    // ???
} else if( $Android ) {
   echo '<a href="whatsapp://send?text=Hello%20World!">Hello, world!</a>';
}
?>