Custom function to separate phones and tablets with wp_is_mobile() function

mobile_tablet.php

<?php
/**
 * It's just an example, proof of concept.
 * You should modify it to include more tablets, not only iPad.
 */ 
function my_wp_is_mobile() {

    static $is_mobile;

    if ( isset($is_mobile) )
        return $is_mobile;

    if ( empty($_SERVER['HTTP_USER_AGENT']) ) {
        $is_mobile = false;
    } elseif (
        strpos($_SERVER['HTTP_USER_AGENT'], 'Android') !== false
        || strpos($_SERVER['HTTP_USER_AGENT'], 'Silk/') !== false
        || strpos($_SERVER['HTTP_USER_AGENT'], 'Kindle') !== false
        || strpos($_SERVER['HTTP_USER_AGENT'], 'BlackBerry') !== false
        || strpos($_SERVER['HTTP_USER_AGENT'], 'Opera Mini') !== false ) {
            $is_mobile = true;
    } elseif (strpos($_SERVER['HTTP_USER_AGENT'], 'Mobile') !== false && strpos($_SERVER['HTTP_USER_AGENT'], 'iPad') == false) {
            $is_mobile = true;
    } elseif (strpos($_SERVER['HTTP_USER_AGENT'], 'iPad') !== false) {
        $is_mobile = false;
    } else {
        $is_mobile = false;
    }

    return $is_mobile;
    
}

See also Mobble plugin:
https://wordpress.org/plugins/mobble/

It gives you some useful functions:

is_handheld(); // any handheld device (phone, tablet, Nintendo)
is_mobile(); // any type of mobile phone (iPhone, Android, Nokia etc)
is_tablet(); // any tablet device (currently iPad, Galaxy Tab)
is_ios(); // any Apple device (iPhone, iPad, iPod) 

Just for a reference, here's original function in WP core:
https://core.trac.wordpress.org/browser/tags/4.2.2/src/wp-includes/vars.php#L0

for anyone who is looking to get this done in 2023:

$tablet_browser = 0;
$mobile_browser = 0;
if (preg_match('/(tablet|ipad|playbook)|(android(?!.*(mobi|opera mini)))/i', strtolower($_SERVER['HTTP_USER_AGENT']))) {
    $tablet_browser++;
}
if (preg_match('/(up.browser|up.link|mmp|symbian|smartphone|midp|wap|phone|android|iemobile)/i', strtolower($_SERVER['HTTP_USER_AGENT']))) {
    $mobile_browser++;
}
if ((strpos(strtolower($_SERVER['HTTP_ACCEPT']),'application/vnd.wap.xhtml+xml') > 0) or ((isset($_SERVER['HTTP_X_WAP_PROFILE']) or isset($_SERVER['HTTP_PROFILE'])))) {
    $mobile_browser++;
}
$mobile_ua = strtolower(substr($_SERVER['HTTP_USER_AGENT'], 0, 4));
$mobile_agents = array(
    'w3c ','acs-','alav','alca','amoi','audi','avan','benq','bird','blac',
    'blaz','brew','cell','cldc','cmd-','dang','doco','eric','hipt','inno',
    'ipaq','java','jigs','kddi','keji','leno','lg-c','lg-d','lg-g','lge-',
    'maui','maxo','midp','mits','mmef','mobi','mot-','moto','mwbp','nec-',
    'newt','noki','palm','pana','pant','phil','play','port','prox',
    'qwap','sage','sams','sany','sch-','sec-','send','seri','sgh-','shar',
    'sie-','siem','smal','smar','sony','sph-','symb','t-mo','teli','tim-',
    'tosh','tsm-','upg1','upsi','vk-v','voda','wap-','wapa','wapi','wapp',
    'wapr','webc','winw','winw','xda ','xda-');
 
if (in_array($mobile_ua,$mobile_agents)) {
    $mobile_browser++;
}
 
if (strpos(strtolower($_SERVER['HTTP_USER_AGENT']),'opera mini') > 0) {
    $mobile_browser++;
    //Check for tablets on opera mini alternative headers
    $stock_ua = strtolower(isset($_SERVER['HTTP_X_OPERAMINI_PHONE_UA'])?$_SERVER['HTTP_X_OPERAMINI_PHONE_UA']:(isset($_SERVER['HTTP_DEVICE_STOCK_UA'])?$_SERVER['HTTP_DEVICE_STOCK_UA']:''));
    if (preg_match('/(tablet|ipad|playbook)|(android(?!.*mobile))/i', $stock_ua)) {
      $tablet_browser++;
    }
}
if ($tablet_browser > 0) {
   // do something for tablet devices
   print 'is tablet';
}
else if ($mobile_browser > 0) {
   // do something for mobile devices
   print 'is mobile';
}
else {
   // do something for everything else
   print 'is desktop';
}   
?>