Understanding Function Scope¶

Now we have the query, but we don’t have the $pdo object. It’s created and lives in get_pets(), but we can’t access it here. Each function is its own little universe and you only have access to the arguments passed in and any variables you create in that function.

Let’s see what I mean. Create a $test variable outside of the get_pet() function and then try to dump it inside of it:

// lib/functions.php
// ...

$test = 'works?';
function get_pet($id)
{
    var_dump($test);die;
    $query = 'SELECT * FROM pet WHERE id = '.$id;
}

Yep, that explodes! The variable $test does not exist. Like I said, each function is its own, isolated universe. When you talk about what variables you have access to and why, we often use the word “scope”. If I say something about the “function scope”, I’m talking about all of the variables that my function has access to. It’s just another one of those techy terms that really describe something simple. Now we can remove this test variable.

We know that copying and pasting the code from get_pets() into get_pet() is code duplication and a recipe for future problems. So how can we get the $pdo variable in get_pet()?

One answer is to make it accessible anywhere by moving that stuff to a new function called get_connection. Let’s do that:

function get_connection()
{
    $config = require 'config.php';

    $pdo = new PDO(
        $config['database_dsn'],
        $config['database_user'],
        $config['database_pass']
    );

    return $pdo;
}

With variables, you have to worry about scope and what you have access to. But functions can be called from anywhere. Call this function from get_pets():

function get_pets($limit = null)
{
    $pdo = get_connection();
    // ... all the same logic
}

Now, use it again from inside get_pet(). To finish this, use the same query() function we saw before. But at the end, use fetch() instead of fetchAll():

function get_pet($id)
{
    $pdo = get_connection();
    $query = 'SELECT * FROM pet WHERE id = '.$id;
    $result = $pdo->query($query);

    return $result->fetch();
}

Use fetchAll to return multiple rows from the database and use fetch when you just want one row, like here.

Refresh! It works! I never doubted you. This looks good, but we have to address our big security hole next.