Source

File: wp-includes/functions.php

function wp_get_update_php_url() {
	$default_url = wp_get_default_update_php_url();

	$update_url = $default_url;
	if ( false !== getenv( 'WP_UPDATE_PHP_URL' ) ) {
		$update_url = getenv( 'WP_UPDATE_PHP_URL' );
	}

	/**
	 * Filters the URL to learn more about updating the PHP version the site is running on.
	 *
	 * Providing an empty string is not allowed and will result in the default URL being used. Furthermore
	 * the page the URL links to should preferably be localized in the site language.
	 *
	 * @since 5.1.0
	 *
	 * @param string $update_url URL to learn more about updating PHP.
	 */
	$update_url = apply_filters( 'wp_update_php_url', $update_url );

	if ( empty( $update_url ) ) {
		$update_url = $default_url;
	}

	return $update_url;
}