Use proper SQL querying

This code uses raw $_GET parameters in sequel queries without escaping them creating a hole for a potential SQL injection exploit.

https://cgit.drupalcode.org/simple_glossary/tree/src/Controller/Glossary...

if((isset($_GET['keyword'])) && (!empty($_GET['keyword'])) ) {
			$keywordLength = strlen($_GET['keyword']);
			if (( $keywordLength == 1 ) && (preg_match("/^[a-zA-Z]$/", strtoupper($_GET['keyword'])))) {
				$whereCondition  = "where (SUBSTR(term,1,1) = '".$_GET['keyword']."') ";  
			} else {
				$whereCondition  =  "where (term LIKE '%".$_GET['keyword']."%')"; 
			}
		} else if((isset($ltr)) && (!empty($ltr)) ) {
			$whereCondition  = "where (SUBSTR(term,1,1) = '".$ltr."')";  
		} else {
			$whereCondition  = "";  
		}
		$result  = db_query("SELECT * FROM simple_glossary_content ".$whereCondition." order by term ASC");

You should be using the Request object to inspect parameters and/or filtering/escaping them. You should also use query placeholders.

See:

• https://www.drupal.org/docs/8/api/database-api/static-queries
• https://www.drupal.org/forum/support/post-installation/2015-05-15/docume...
• https://drupal.stackexchange.com/questions/201549/how-do-i-add-a-conditi...
• https://drupal.stackexchange.com/questions/70641/placeholders-and-the-qu...