http://php.net/manual/fr/function.htmlentities.php
Ma fonction:

function secure($variable)
	    {
	        $variable = htmlentities($variable, ENT_QUOTES, "UTF-8");
	        return $variable;
	    }

Utilisation:
 $query = $this->bdd->query("SELECT * FROM user WHERE Pseudo='".$this->secure($pseudo)."' AND Password='".$this->secure(md5($password))."'");