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.

  • Static. true/false. $installing

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.
Default: null

Code of wp installing: wp-includes/load.php VER 5.1.1

<?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;
}