StringClassReferenceSniff.php

<?php declare(strict_types = 1);

namespace MyCodingStandard\Sniffs\Formatting;

use PHP_CodeSniffer\Sniffs\Sniff;
use PHP_CodeSniffer\Files\File;

class StringClassReferenceSniff implements Sniff
{
    public function register()
    {
        return [T_CONSTANT_ENCAPSED_STRING];
    }

    public function process(File $phpcsFile, $stackPtr)
    {
        $tokens = $phpcsFile->getTokens();
        if (strpos($tokens[$stackPtr]['content'], '\\') !== false) {
            // Remove quotes from string
            $className = str_replace(['"', "'"], '', $tokens[$stackPtr]['content']);
            if (class_exists($className) || interface_exists($className)) {
                $error = 'String containing class reference found, use ::class instead.';
                $data  = [trim($tokens[$stackPtr]['content'])];
                $phpcsFile->addError($error, $stackPtr, 'Found', $data);
            }
        }
    }
}