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<?php
// Simple way
add_filter( 'show_admin_bar', '__return_false' );
// Standard way
add_filter( 'show_admin_bar', 'show_admin_bar_cb' );
function show_admin_bar_cb() {
  return false;
}
// Hide admin bar for all users except admin
add_filter( 'show_admin_bar', 'show_admin_bar_cb' );
function show_admin_bar_cb() {
  if( is_super_admin() )
    return true;
  return false;
}
// Hide admin bar for all users except admin (shorter version)
add_filter( 'show_admin_bar', 'show_admin_bar_cb' );
function show_admin_bar_cb() {
  return is_super_admin();
}
// Hide admin bar for subscribers only
add_filter( 'show_admin_bar', 'show_admin_bar_cb' );
function show_admin_bar_cb() {
  $user = wp_get_current_user();
  if( in_array( "subscriber", (array) $user->roles ) ) {
    return false
  }
  return true;
}
// Hide admin bar for subscribers only (shorter version)
add_filter( 'show_admin_bar', 'show_admin_bar_cb' );
function show_admin_bar_cb() {
  $user = wp_get_current_user();
  return ! in_array( "subscriber", (array) $user->roles );
}