MSSQL is designed for more financially-based websites, and not apps.
PHP is widely used alongside MySQL, so it must be good if "the" largest site on the web uses it (Google fyi).
More sensitive apps use Oracle or MSSQL.
Don't forget, Facebook was built using PHP on a Oracle backend since 2004, because they handle 200 million+ users they need it. MySQL in huge volume (millions we're talking) starts to becomes expensive to keep running at a fast speed.
Even some financial services use MySQL, including PayPal.
