Partial POC

test.php

<?php

function sum(int $a, int $b, &$c) {
    $c = $a + $b;
    return $c;
}

function reflection_param_to_string(\ReflectionParameter $reflectionParam, bool $useList = false) {
    $string = '';

    if (!$useList && $reflectionParam->hasType()) {
        $string .= (string) $reflectionParam->getType() . ' ';
    }

    if ($reflectionParam->isPassedByReference()) {
        $string .= '&';
    }

    $string .= '$' . $reflectionParam->getName();

    if (!$useList && $reflectionParam->isDefaultValueAvailable()) {
        $string .= ' = ' . var_export($reflectionParam->getDefaultValue(), true);
    }

    return $string;
}

function create_partial_closure($callable, array $placeholderPositions)
{
    $reflection = new ReflectionFunction($callable);
    $reflectionParams = $reflection->getParameters();

    $outerParamList = [];
    $innerParamList = [];
    $innerUseList = [];
    $innerArgList = [];

    foreach ($reflectionParams as $i => $reflectionParam) {
        $paramString = reflection_param_to_string($reflectionParam);
        if (in_array($i, $placeholderPositions, true)) {
            $outerParamList[] = $paramString;
            $innerUseList[] = reflection_param_to_string($reflectionParam, true);
        } else {
            $innerParamList[] = $paramString;
        }

        $innerArgList[] = '$' . $reflectionParam->getName();
    }

    $outerParamString = implode(', ', $outerParamList);
    $innerParamString = implode(', ', $innerParamList);
    $innerUseString = implode(', ', $innerUseList);
    $innerArgString = implode(', ', $innerArgList);

    $eval = <<<PHP
        return function (callable \$callable, $outerParamString) {
            return function ($innerParamString) use (\$callable, $innerUseString) {
                return \$callable($innerArgString);
            };
        };
    PHP;

    return eval($eval);
}

$c = null;
$partial_call = create_partial_closure('sum', [1, 2]);
$partial_call_intermediate = $partial_call('sum', 1, $c);
$result = $partial_call_intermediate(10);

var_dump($c);
var_dump($result);