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I am switching from plain mysql in php to PDO and I have noticed that the common way to test for errors is using a try / catch combination instead of if / else combinations.
What is the advantage of that method, can I use one try / catch block instead of several nested if / else blocks to handle all errors for the different steps (connect, prepare, execute, etc.)?
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In 'try-catch' the codes to handle the exceptions and what exception to be handled, that are easily readable. In 'if-else', we have one else block corresponding to one if block. Or we need to define another condition with command 'else if'. In 'try-catch' we don't have to define each 'try' block with a 'catch' block.
Try-catch block can be used to handle system generated errors as well as to implement conditional statements by manually raising exceptions while if else block can only implement conditional statements and cannot handle system generated errors.
If you've one if/else block instead of one try/catch block, and if an exceptions throws in the try/catch block, then the if/else block is faster (if/else block: around 0.0012 milliseconds, try/catch block: around 0.6664 milliseconds). If no exception is thrown with a try/catch block, then a try/catch block is faster.
With a try catch, you can handle an exception that may include logging, retrying failing code, or gracefully terminating the application. Without a try catch, you run the risk of encountering unhandled exceptions. Try catch statements aren't free in that they come with performance overhead.
I'd use the try/catch block when the normal path through the code should proceed without error unless there are truly some exceptional conditions -- like the server being down, your credentials being expired or incorrect. I wouldn't necessarily use it to handle non-exceptional errors -- say like the current user not being in the correct role. That is, when you can reasonably expect and handle an error that is not an exceptional condition, I think you should do your checks.
In the case that you've described -- setting up and performing a query, a try/catch block is an excellent way to handle it as you normally expect the query to succeed. On the other hand, you'll probably want to check that the contents of result are what you expect with control flow logic rather than just attempting to use data that may not be valid for your purpose.
One thing that you want to look out for is sloppy use of try/catch. Try/catch shouldn't be used to protect yourself from bad programming -- the "I don't know what will happen if I do this so I'm going to wrap it in a try/catch and hope for the best" kind of programming. Typically you'll want to restrict the kinds of exceptions you catch to those that are not related to the code itself (server down, bad credentials, etc.) so that you can find and fix errors that are code related (null pointers, etc.).
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In general, try-catch blocks are great because they will break (move to the catch statement) whenever the exception occurs. If-else blocks rely on you predicting when the error will happen.
Edit: Also, catch blocks won't stop your code from halting when an error is hit.
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