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Handling Exceptions
4:23What happens when the system is unable to connect to the database? Whenever your code relies on an external system, there's always the chance that something could go wrong. These are known as exceptions and any code connecting to an external system needs to handle them.
Whenever your code relies
on an external system
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there's always the chance that
something could go wrong.
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What if the database is down?
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How should your code respond?
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These are known as exceptions and
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any code connecting to an external
system needs to handle them.
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Managing errors in any
application is very important
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especially when it comes
to database errors.
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Remember, exceptions are exceptional.
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You shouldn't use them with code
that you're expecting to happen.
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If the code is functioning as designed,
it should never hit an exception.
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However if the code encounters
something that will not
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allow it to function as intended,
then we want to catch that exception, and
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most likely, log it, and
notify someone in some way.
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Go back into the working
file connection.php.
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Here we're creating a new pdo object
that connects to our database.
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Any code that connects to an external
system should be placed inside a block of
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code that can handle exceptions.
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We can't always control external systems.
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So it's a good idea to make sure
we can still get the data we need.
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In php the block of code for handling an
exception is called the try catch block.
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Before the code we want to try to execute,
you put a try command.
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You surround the code you want to
try with a set of curly bracket.
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Php will try to execute all the code
inside those curly brackets.
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You then put a catch command
after the try block.
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If an exception occurs php will execute
the code inside this catch block.
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After the catch,
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you type a set of parenthesis
with the following code inside.
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Exception e.
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When the exception occurs an exception
will get passed to the catch block.
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This is actually an object instantiating
phps native exception class.
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The way the exception gets
passed to the catch block
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is somewhat similar to how an argument
gets passed to a function.
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And this syntax with parentheses
looks a bit like that.
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Inside the catch block the variable e will
contain the details about the exception.
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After this we'll put another
set of curly brackets.
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If there is an exception the code inside
these curly brackets will get executed.
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After this let's just stop any more code
from executing with the exit command.
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Just to recap.
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This block of code tries to create
an object from the pto class.
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Connected to our database.
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If there's a problem,
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an exception is thrown in the code
inside a catch block is executed.
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This message here is displayed and
our code exits.
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If everything connects just fine
the catch block will be skipped.
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And any code after this block
would execute just fine.
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Remember, the try catch block
is not an if then statement.
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We don't want the code in
the catch block to ever be run.
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Let's display a message down.
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Now let's see what happens if
the database is misspelled.
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We'll add a couple extra A's,
and save the file.
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Now, we'll take a look
at this in our browser.
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Since the information needed to
connect to the database is incorrect,
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an exception is thrown.
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The same thing would happen if
the database server were offline.
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The code in the catch block echoes
out the message, and exits.
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Let's go back and
fix the connection string.
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Now, in the browser you can see that
the code to display the information about
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the pdo object is executed from inside
the try portion of our try catch block.
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We also see the positive
message after that block.
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Because there is not an exception thrown,
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the code inside the catch
block is never run.
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Before we continue, we're going to do
two more things to our pdo object.
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We want to make sure that
we're aware of all issues and
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make sure that those issues return as
much useful information as possible.
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This will help us fix
those issues right away.
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