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<?php

function ccd_change_howdy( $translated, $text, $domain ) {
    
    // Don't change on the front end.
    if ( ! is_admin() || 'default' != $domain ) {
        return $translated;
    }
  
    // Replace "Howdy" with "Welcome" on the back end.
    if ( false !== strpos( $translated, 'Howdy' ) ) {
        return str_replace( 'Howdy', 'Welcome', $translated );
    }

    return $translated;
}
add_filter( 'gettext', 'ccd_change_howdy', 10, 3 );