wpdb->insert with special chars failing with collation utf8mb4_unicode_520_ci

Use the sanitize field

sanitize_text_field($_POST['c_name']);

more info https://developer.wordpress.org/reference/functions/sanitize_text_field/

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)