// Created on savesnippets.com · https://savesnippets.com/hFWnr0ELyG1GrO
<?php
function setPath(array &$arr, string $path, mixed $value): void {
    $cursor = &$arr;
    $segs   = explode('.', $path);
    $last   = array_pop($segs);
    foreach ($segs as $seg) {
        if (!isset($cursor[$seg]) || !is_array($cursor[$seg])) {
            $cursor[$seg] = [];
        }
        $cursor = &$cursor[$seg];
    }
    $cursor[$last] = $value;
}

$state = [];
setPath($state, 'user.profile.email', 'a@x.com');
setPath($state, 'user.profile.name',  'Alice');
print_r($state);
// [ user => [ profile => [ email => a@x.com, name => Alice ] ] ]