Example snippet showing how to update the WordPress .htaccess file programatically

update_htaccess.php

<?php
/**
 * Update .htaccess to whitelist logo when modified in platform settings.
 */
function cc_logo_htaccess() {
	// Fire only on Platform Settings options page.
	$screen = get_current_screen();
	if ( 'cc-configurator_page_cc-settings' === $screen->id && ( $_POST['acf']['field_576c2a5137029'] || $_POST['acf']['field_576c2a9e3702a'] ) ) {

		// Store files in an array.
		$whitelisted_files = array();

		// Get Platform and Client logos.
		if ( ! empty( $_POST['acf']['field_576c2a5137029'] ) ) { // cc logo.
			$cc_logo = basename( wp_get_attachment_url( $_POST['acf']['field_576c2a5137029'] ) );
			$cc_logo_parts = pathinfo( $cc_logo );
			$whitelisted_files[] = preg_quote( $cc_logo_parts['filename'] ) . '.*?' . preg_quote( '.' . $cc_logo_parts['extension'] );
		}
		if ( ! empty( $_POST['acf']['field_576c2a9e3702a'] ) ) { // Client logo.
			$client_logo = basename( wp_get_attachment_url( $_POST['acf']['field_576c2a9e3702a'] ) );
			$client_logo_parts = pathinfo( $client_logo );
			$whitelisted_files[] = preg_quote( $client_logo_parts['filename'] ) . '.*?' . preg_quote( '.' . $client_logo_parts['extension'] );
		}

		// If we have files, build string and update .htaccess.
		if ( ! empty( $whitelisted_files ) ) {
			$whitelist = '<FilesMatch "(' . implode( '|', $whitelisted_files ) . ')$">
				Satisfy Any
				Order allow,deny
				Allow from all
				Deny from none
			</FilesMatch>';

			$success = insert_with_markers( trailingslashit( get_home_path() ) . '.htaccess', 'CC Logo whitelist', $whitelist );
			if ( false === $success ) {
				error_log( 'ERROR:: .htaccess for "' . get_bloginfo( 'name' ) . '" could not be updated with logo whitelist.' );
			}
		}

	}
}
add_action( 'acf/save_post', 'cc_logo_htaccess' );