Source

File: wp-admin/includes/class-wp-list-table.php

public function print_column_headers( $with_id = true ) {
		list( $columns, $hidden, $sortable, $primary ) = $this->get_column_info();

		$current_url = set_url_scheme( 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'] );
		$current_url = remove_query_arg( 'paged', $current_url );

		if ( isset( $_GET['orderby'] ) ) {
			$current_orderby = $_GET['orderby'];
		} else {
			$current_orderby = '';
		}

		if ( isset( $_GET['order'] ) && 'desc' === $_GET['order'] ) {
			$current_order = 'desc';
		} else {
			$current_order = 'asc';
		}

		if ( ! empty( $columns['cb'] ) ) {
			static $cb_counter = 1;
			$columns['cb']     = '<label class="screen-reader-text" for="cb-select-all-' . $cb_counter . '">' . __( 'Select All' ) . '</label>'
				. '<input id="cb-select-all-' . $cb_counter . '" type="checkbox" />';
			$cb_counter++;
		}

		foreach ( $columns as $column_key => $column_display_name ) {
			$class = array( 'manage-column', "column-$column_key" );

			if ( in_array( $column_key, $hidden, true ) ) {
				$class[] = 'hidden';
			}

			if ( 'cb' === $column_key ) {
				$class[] = 'check-column';
			} elseif ( in_array( $column_key, array( 'posts', 'comments', 'links' ), true ) ) {
				$class[] = 'num';
			}

			if ( $column_key === $primary ) {
				$class[] = 'column-primary';
			}

			if ( isset( $sortable[ $column_key ] ) ) {
				list( $orderby, $desc_first ) = $sortable[ $column_key ];

				if ( $current_orderby === $orderby ) {
					$order = 'asc' === $current_order ? 'desc' : 'asc';

					$class[] = 'sorted';
					$class[] = $current_order;
				} else {
					$order = strtolower( $desc_first );

					if ( ! in_array( $order, array( 'desc', 'asc' ), true ) ) {
						$order = $desc_first ? 'desc' : 'asc';
					}

					$class[] = 'sortable';
					$class[] = 'desc' === $order ? 'asc' : 'desc';
				}

				$column_display_name = sprintf(
					'<a href="%s"><span>%s</span><span class="sorting-indicator"></span></a>',
					esc_url( add_query_arg( compact( 'orderby', 'order' ), $current_url ) ),
					$column_display_name
				);
			}

			$tag   = ( 'cb' === $column_key ) ? 'td' : 'th';
			$scope = ( 'th' === $tag ) ? 'scope="col"' : '';
			$id    = $with_id ? "id='$column_key'" : '';

			if ( ! empty( $class ) ) {
				$class = "class='" . join( ' ', $class ) . "'";
			}

			echo "<$tag $scope $id $class>$column_display_name</$tag>";
		}
	}