backend.php

<?php
while (ob_get_level()) {
    ob_end_clean();
}

header('Access-Control-Allow-Origin: *');
header('Access-Control-Allow-Methods: POST, OPTIONS');
header('Access-Control-Allow-Headers: Content-Type, X-API-Key');
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');
header('Connection: keep-alive');
header('X-Accel-Buffering: no');

if ($_SERVER['REQUEST_METHOD'] === 'OPTIONS') {
    exit(0);
}

$API_KEY = $_SERVER['HTTP_X_API_KEY'] ?? '';

if (empty($API_KEY)) {
    echo "data: " . json_encode(['error' => 'API-Key missing!']) . "\n\n";
    exit;
}

$input = file_get_contents('php://input');

$ch = curl_init('https://api.anthropic.com/v1/messages');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, false);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $input);
curl_setopt($ch, CURLOPT_HTTPHEADER, [
    'x-api-key: ' . $API_KEY,
    'anthropic-version: 2023-06-01',
    'content-type: application/json'
]);

curl_setopt($ch, CURLOPT_WRITEFUNCTION, function($curl, $data) {
    echo $data;
    if (ob_get_level() > 0) ob_flush();
    flush();
    return strlen($data);
});

$result = curl_exec($ch);

if ($result === false) {
    $error_msg = curl_error($ch);
    echo "data: " . json_encode(['error' => 'CURL-Error: ' . $error_msg]) . "\n\n";
} else {
    $http_code = curl_getinfo($ch, CURLINFO_HTTP_CODE);
    if ($http_code >= 400) {
        // ...
    }
}

curl_close($ch);
?>