You should be able to use an Anonymous Function (aka Closure) with a call to the parent scoped $delimiter variable, like so:

$callbacks[$delimiter] = function($matches) use ($delimiter) {
    return $delimiter . strtolower($matches[1]);
};

I would like to contribute with a very simple case I found in a WordPress Theme and seems to work properly:

Having the following add_filter statement:

add_filter(
  'option_page_capability_' . ot_options_id(),
  create_function( '$caps', "return '$caps';" ),
  999
);

Replace it for:

add_filter(
  'option_page_capability_' . ot_options_id(),
  function($caps) {return $caps;},
  999
);

We can see the usage of function(), very typical function creation instead of a deprecated create_function() to create functions.

Automated Upgrade

If anyone needs to upgrade dozens of create_function() cases in their code to anonymous functions, I work on a tool called Rector.

It goes through the code and replaces the create_function with anonymous functions 1:1. It’s tested on 30 various cases.

Install

composer require rector/rector --dev

Setup

Let’s say you want to upgrade code in the /src directory.

# rector.php
<?php

use RectorCoreConfigurationOption;
use SymfonyComponentDependencyInjectionLoaderConfiguratorContainerConfigurator;
use RectorPhp72RectorFuncCallCreateFunctionToAnonymousFunctionRector;

return static function (ContainerConfigurator $containerConfigurator) {
    $parameters = $containerConfigurator->parameters();
    $parameters->set(Option::PATHS, [
        __DIR__ . '/src',
    ]);

    $services = $containerConfigurator->services();
    $services->set(CreateFunctionToAnonymousFunctionRector::class);
};

Run on your code

# this is set run, it only report what it would change
vendor/bin/rector process --config rector.php --dry-run

# this actually changes the code
vendor/bin/rector process --config rector.php

# the "rector.php" config is loaded by default, so we can drop it
vendor/bin/rector process

EDIT: Updated 2020-10-31 with PHP Rector 0.8.x syntax

Since PHP 7.4 you can use an Arrow function:

$callbacks[$delimiter] = fn($matches) => $delimiter . strtolower($matches[1]);

Arrow functions are shorter than anonymous functions, and use the parent scope – so you can refer to $delimiter without passing it in.

This Array of Anonymous functions worked for me, see code below:

// This will be a dynamic name that could 
// be used as a function like "namespace".
$dynamic_name = 'my_dynamic_name';

// Here's some variables that you could use in the scope of
// your dynamic anonymous functions. 
$outerVariable = 'If I need this varible, I can use it';
$outerVariableTwo = 'If I need this varible, I can use it too!';

// Create an array that we can later use and turn into 
// and associative array with our new dynamic anonymous functions.
$dynamicAnonFunctions = [];

// Create the first dynamic function.
$dynamicAnonFunctions[($dynamic_name."_func_one")] = function () use ($outerVariable, $dynamic_name) { 
    echo 'Running: function <b>'.$dynamic_name .'_func_one()</b>';
    echo '<br><br>';
    echo $outerVariable;
    echo '<br><br>';
    echo 'This works :)'; 
    echo '<br><br>';
};

// Create the second dynamic function 
$dynamicAnonFunctions[($dynamic_name."_func_two")] = function () use ($outerVariableTwo, $dynamic_name) { 
    echo '- - - - - - - - - - - - - - - - - - - ';
    echo '<br><br>';
    echo 'Running: function <b>'.$dynamic_name .'_func_two()</b>';
    echo '<br><br>';
    echo $outerVariableTwo;
    echo '<br><br>';
    echo 'This also works :)!'; 
    echo '<br><br>';
};

// Call the functions.
$dynamicAnonFunctions[($dynamic_name."_func_one")]();
$dynamicAnonFunctions[($dynamic_name."_func_two")]();

// Halt execution.
exit();

Just copy this into your script file and you will see the output from the echo statements, then simply remap the function to your own will!

Happy coding =)

The anonymous function solution works, but if the expression to be returned is in a string I think eval should be used.

$callbacks[$delimiter] = eval('return function($matches){return '.$delimiter.' . strtolower($matches[1]);};');