Source

File: wp-includes/user.php

function get_password_reset_key( $user ) {
	global $wp_hasher;

	if ( ! ( $user instanceof WP_User ) ) {
		return new WP_Error( 'invalidcombo', __( '<strong>Error</strong>: There is no account with that username or email address.' ) );
	}

	/**
	 * Fires before a new password is retrieved.
	 *
	 * Use the {@see 'retrieve_password'} hook instead.
	 *
	 * @since 1.5.0
	 * @deprecated 1.5.1 Misspelled. Use {@see 'retrieve_password'} hook instead.
	 *
	 * @param string $user_login The user login name.
	 */
	do_action_deprecated( 'retreive_password', array( $user->user_login ), '1.5.1', 'retrieve_password' );

	/**
	 * Fires before a new password is retrieved.
	 *
	 * @since 1.5.1
	 *
	 * @param string $user_login The user login name.
	 */
	do_action( 'retrieve_password', $user->user_login );

	$allow = true;
	if ( is_multisite() && is_user_spammy( $user ) ) {
		$allow = false;
	}

	/**
	 * Filters whether to allow a password to be reset.
	 *
	 * @since 2.7.0
	 *
	 * @param bool $allow Whether to allow the password to be reset. Default true.
	 * @param int  $ID    The ID of the user attempting to reset a password.
	 */
	$allow = apply_filters( 'allow_password_reset', $allow, $user->ID );

	if ( ! $allow ) {
		return new WP_Error( 'no_password_reset', __( 'Password reset is not allowed for this user' ) );
	} elseif ( is_wp_error( $allow ) ) {
		return $allow;
	}

	// Generate something random for a password reset key.
	$key = wp_generate_password( 20, false );

	/**
	 * Fires when a password reset key is generated.
	 *
	 * @since 2.5.0
	 *
	 * @param string $user_login The username for the user.
	 * @param string $key        The generated password reset key.
	 */
	do_action( 'retrieve_password_key', $user->user_login, $key );

	// Now insert the key, hashed, into the DB.
	if ( empty( $wp_hasher ) ) {
		require_once ABSPATH . WPINC . '/class-phpass.php';
		$wp_hasher = new PasswordHash( 8, true );
	}

	$hashed = time() . ':' . $wp_hasher->HashPassword( $key );

	$key_saved = wp_update_user(
		array(
			'ID'                  => $user->ID,
			'user_activation_key' => $hashed,
		)
	);

	if ( is_wp_error( $key_saved ) ) {
		return $key_saved;
	}

	return $key;
}