diff --git a/AGENTS.md b/AGENTS.md
index 3ce1694..e1c2dc4 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -2,9 +2,11 @@
 
 ## Project Structure & Module Organization
 
-This repository contains a small ESP8266/ESP32 Arduino project for monitoring water level with conductive electrodes.
+This repository contains a small ESP8266/ESP32 Arduino project for monitoring water level with conductive electrodes and a small PHP endpoint for logging measurements.
 
 - `arduino/arduino.ino` contains the Arduino sketch and main application logic.
+- `server/log.php` accepts measurement POST requests and appends them as JSON Lines.
+- `data/YYYY.jsonl` is created at runtime for logged measurements and should not be treated as source code.
 - `README.md` gives the high-level project purpose.
 - `LICENSE` contains licensing information.
 - `.agents/` is reserved for agent/tooling metadata and should not be treated as firmware source.
@@ -25,11 +27,14 @@ arduino-cli monitor -p COM3 -c baudrate=115200
 arduino-cli compile --fqbn esp32:esp32:esp32 arduino
 arduino-cli upload -p COM3 --fqbn esp32:esp32:esp32 arduino
 arduino-cli monitor -p COM3 -c baudrate=115200
+
+php -l server/log.php
 ```
 
 - `compile` verifies the sketch builds for the selected board.
 - `upload` flashes the firmware to the connected board.
 - `monitor` opens the serial console used by debug output.
+- `php -l` checks the PHP logging endpoint for syntax errors.
 
 Adjust `--fqbn` and `COM3` for the actual board and port.
 
@@ -43,6 +48,8 @@ Write C++ compatible with Arduino IDE and the ESP8266/ESP32 cores. Use two-space
 
 Prefer small functions with one responsibility. Keep hardware pins, URLs, timings, and debug flags as constants near the top of the sketch.
 
+For PHP scripts, keep them framework-free unless the project grows. Return JSON responses, use explicit HTTP status codes, and keep generated log files under `data/`.
+
 ## Hardware Notes
 
 For ESP8266 conductive level sensing, use external pull-down resistors from each `PIN_LEVEL1..4` input to `GND`. The common electrode is driven to positive voltage only during measurement.
@@ -60,6 +67,13 @@ Before committing firmware changes:
 - Test `DEBUG == 2` for repeated Wi-Fi POST requests.
 - Test `DEBUG == 0` for production sleep/wake behavior.
 
+Before committing server changes:
+
+- Run `php -l server/log.php`.
+- Test that non-POST requests return HTTP 405 with `Allow: POST`.
+- Test a form POST such as `level1=1&level2=0&level3=0&level4=1`.
+- Verify that a single compact JSON object is appended to `data/YYYY.jsonl`.
+
 ## Commit & Pull Request Guidelines
 
 Current Git history is minimal and uses a simple message such as `Initial commit`. Keep future commits short, imperative, and specific, for example `Add WiFi timeout handling`.
diff --git a/server/log.php b/server/log.php
new file mode 100644
index 0000000..04799c9
--- /dev/null
+++ b/server/log.php
@@ -0,0 +1,88 @@
+<?php
+
+declare(strict_types=1);
+
+function respondJson(int $statusCode, array $payload): void
+{
+	http_response_code($statusCode);
+	header('Content-Type: application/json; charset=utf-8');
+	echo json_encode($payload) . PHP_EOL;
+}
+
+if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
+	header('Allow: POST');
+	respondJson(405, [
+		'success' => false,
+		'error' => 'Method Not Allowed',
+	]);
+	exit;
+}
+
+$levels = [];
+foreach (['level1', 'level2', 'level3', 'level4'] as $name) {
+	if (!array_key_exists($name, $_POST)) {
+		$levels[$name] = 0;
+		continue;
+	}
+
+	if (is_array($_POST[$name])) {
+		respondJson(400, [
+			'success' => false,
+			'error' => 'Invalid value for ' . $name,
+		]);
+		exit;
+	}
+
+	$value = trim((string) $_POST[$name]);
+	if ($value !== '0' && $value !== '1') {
+		respondJson(400, [
+			'success' => false,
+			'error' => 'Invalid value for ' . $name,
+		]);
+		exit;
+	}
+
+	$levels[$name] = (int) $value;
+}
+
+$record = [
+	'timestamp' => date('c'),
+	'level1' => (int) $levels['level1'],
+	'level2' => (int) $levels['level2'],
+	'level3' => (int) $levels['level3'],
+	'level4' => (int) $levels['level4'],
+];
+
+$dataDir = __DIR__ . DIRECTORY_SEPARATOR . '..' . DIRECTORY_SEPARATOR . 'data';
+if (!is_dir($dataDir)) {
+	if (!mkdir($dataDir, 0755, true) && !is_dir($dataDir)) {
+		respondJson(500, [
+			'success' => false,
+			'error' => 'Failed to create data directory',
+		]);
+		exit;
+	}
+}
+
+$filename = $dataDir . DIRECTORY_SEPARATOR . date('Y') . '.jsonl';
+$json = json_encode($record);
+if ($json === false) {
+	respondJson(500, [
+		'success' => false,
+		'error' => 'Failed to encode record',
+	]);
+	exit;
+}
+
+$written = file_put_contents($filename, $json . PHP_EOL, FILE_APPEND | LOCK_EX);
+if ($written === false) {
+	respondJson(500, [
+		'success' => false,
+		'error' => 'Failed to write log file',
+	]);
+	exit;
+}
+
+respondJson(200, [
+	'success' => true,
+]);