WordPress at a glance
function is not described

wp_installing() WP 4.4.0

Check or set whether WordPress is in "installation" mode.

If the WP_INSTALLING constant is defined during the bootstrap, wp_installing() will default to true.

No Hooks.

Return

true|false. True if WP is installing, otherwise false. When a $is_installing is passed, the function will report whether WP was in installing mode prior to the change to $is_installing.

Usage

wp_installing( $is_installing );
$is_installing(true|false)
True to set WP into Installing mode, false to turn Installing mode off. Omit this parameter if you only want to fetch the current status.

Changelog

Since 4.4.0 Introduced.

Code of wp_installing() WP 5.7

<?php
function wp_installing( $is_installing = null ) {
	static $installing = null;

	// Support for the `WP_INSTALLING` constant, defined before WP is loaded.
	if ( is_null( $installing ) ) {
		$installing = defined( 'WP_INSTALLING' ) && WP_INSTALLING;
	}

	if ( ! is_null( $is_installing ) ) {
		$old_installing = $installing;
		$installing     = $is_installing;
		return (bool) $old_installing;
	}

	return (bool) $installing;
}