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<?php

//////////////////
// CHANGE HOWDY //
//////////////////

function change_howdy( $wp_admin_bar ) {
    $my_account = $wp_admin_bar->get_node('my-account');
    $newtext = str_replace( 'Howdy,', 'Hi,', $my_account->title );
    $wp_admin_bar->add_node( array(
        'id' => 'my-account',
        'title' => $newtext,
    ) );
}
add_filter( 'admin_bar_menu', 'change_howdy', 25 );