Strain testing

My website frequently seems to be hugely slow or chuck out loads of php/mysql errors. Initially i thought this the fault of my web hoster, but they're so big and popular I can't believe this is caused by them. Which leads me to think maybe the problems are caused when a certain number of people are trying to access the page simultaneously? How can I test for this? 1) Is there any site/software that will perform a "strain test" or similar? 2) What precautions should I take in dealing with my mysql database to minimise any problems? 3) What precautions should I take in dealing with my php scripts to minimise any problems? For example. Each page I have loads a header.php4 and a footer.php4 with the content inbetween. I open a MySQL connection in header.php4 and close it in footer.php4. Is this good practice, or should I open and close it as and when I need it? 90% of my pages require database interaction so it seemed like a good idea at the time...... - seb --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.274 / Virus Database: 144 - Release Date: 23/08/2001