Replacing and resolving variables inside a PHP string

If you had a string that looked something like this:

Hello! My name is {$name}, I am {$age} years old.

…and you needed to replace all those variables with something dynamic. Here’s how you can do it:

Alternative 1: Pre-defined array

We can pre-define a replacement array of what we want to exchange and what we want to change things into.

$string = 'Hello! My name is {$name}, I am {$age} years old.';
$replacement_array = array(
    'name' => 'Bob',
    'age' => 22
);

$string_processed = preg_replace_callback(
    '~\{\$(.*?)\}~si',
    function($match) use ($replacement_array)
    {
        return str_replace($match[0], isset($replacement_array[$match[1]]) ? $replacement_array[$match[1]] : $match[0], $match[0]);
    },
    $string);

echo $string_processed;

This script will print:

Hello! My name is Bob, I am 22 years old.

Of course the $replacement_array can contain any values you want, even dynamic ones. But perhaps a pre-defined array is too much work, let’s try another approach:

Alternative 2: Resolve the variables from global space (or whatever scope you happen to be in at the time of execution).

$string = 'Hello! My name is {$name}, I am {$age} years old.';
$name = "Bob";
$age = 22;

$string_processed = preg_replace_callback(
    '~\{\$(.*?)\}~si',
    function($match) use ($name, $age)
    {
        return eval('return $' . $match[1] . ';');
    },
    $string);

echo $string_processed;

No more replacement array! But because we use preg_replace_callback with an anonymous function, we have to declare that we are going to “use” the variables $name and $age inside the callback function.

In effect, this is kind of like a tiny templating engine. :)

Source of regular expression