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When you're using prepared statements you pass the query string to prepare(). You don't use query() at all.

Your math is off. There are 1,000 mm in a meter. But there are 1,000,000 square mm in a square meter. You should divide by 1,000,000 if you use the approach in your original post. 900mm * 1500mm / 1000000 = 1.35 sq meters

So then NotSunFighter had the right approach. And using the number_format function would allow you to adjust the presentation.

I believe I would convert the mm entrees to meters first then get the square meters. $dimension1 = 850/1000; $dimension2 = 1447/1000; $square_metres = $dimension1 * $dimension2; The results would be 1.22995 square meters. You can use number_format() if you need to limit the number of decimal places.

Your RegEx for email addresses is already flawed. Use built-in methods unless, as @gizmola stated, you have a use case that is not supported). As to your second question, if the error message you are referring to is one that is thrown from the DB, then you should absolutely have logic to show the message you define. Never expose DB errors to the user. You should always capture those and show a "friendly" message to the user that does not expose any details they could use to infiltrate the system.