diff --git a/src/main/php/io/streams/Buffer.class.php b/src/main/php/io/streams/Buffer.class.php
new file mode 100755
index 000000000..5e76a7bef
--- /dev/null
+++ b/src/main/php/io/streams/Buffer.class.php
@@ -0,0 +1,124 @@
+<?php namespace io\streams;
+
+use io\{File, Folder};
+use lang\{IllegalArgumentException, IllegalStateException};
+
+/**
+ * Buffers in memory up until a given threshold, using the file system once
+ * it's exceeded.
+ * 
+ * @see   https://github.com/xp-forge/web/issues/118
+ * @test  io.unittest.BufferTest
+ */
+class Buffer implements InputStream, OutputStream {
+  private $files, $threshold;
+  private $memory= '';
+  private $file= null;
+  private $size= 0;
+  private $pointer= 0;
+  private $draining= false;
+
+  /**
+   * Creates a new buffer
+   *
+   * @param  io.Folder|io.Path|string $files
+   * @param  int $threshold
+   * @throws lang.IllegalArgumentException
+   */
+  public function __construct($files, int $threshold) {
+    if ($threshold < 0) {
+      throw new IllegalArgumentException('Threshold must be >= 0');
+    }
+
+    $this->files= $files instanceof Folder ? $files->getURI() : (string)$files;
+    $this->threshold= $threshold;
+  }
+
+  /** Returns buffer size */
+  public function size(): int { return $this->size; }
+
+  /** Returns the underlying file, if any */
+  public function file(): ?File { return $this->file; }
+
+  /** Returns whether this buffer is draining */
+  public function draining(): bool { return $this->draining; }
+
+  /**
+   * Write a string
+   *
+   * @param  var $arg
+   * @return void
+   * @throws lang.IllegalStateException
+   */
+  public function write($bytes) {
+    if ($this->draining) throw new IllegalStateException('Started draining buffer');
+
+    $this->size+= strlen($bytes);
+    if ($this->size <= $this->threshold) {
+      $this->memory.= $bytes;
+      return;
+    }
+
+    if (null === $this->file) {
+      $this->file= new File(tempnam($this->files, "b{$this->threshold}"));
+      $this->file->open(File::READWRITE);
+      $this->file->write($this->memory);
+      $this->memory= null;
+    }
+    $this->file->write($bytes);
+  }
+
+  /** @return void */
+  public function flush() {
+    $this->file && $this->file->flush();
+  }
+
+  /**
+   * Resets buffer to be able to read from the beginning
+   *
+   * @return  void
+   */
+  public function reset() {
+    $this->file ? $this->file->seek(0, SEEK_SET) : $this->pointer= 0;
+    $this->draining= true;
+  }
+
+  /** @return int */
+  public function available() {
+    return $this->draining
+      ? $this->size - ($this->file ? $this->file->tell() : $this->pointer)
+      : $this->size
+    ;
+  }
+
+  /**
+   * Read a string
+   *
+   * @param  int $limit
+   * @return string
+   */
+  public function read($limit= 8192) {
+    if ($this->file) {
+      $this->draining || $this->file->seek(0, SEEK_SET) && $this->draining= true;
+      return (string)$this->file->read($limit);
+    } else {
+      $this->draining= true;
+      $chunk= substr($this->memory, $this->pointer, $limit);
+      $this->pointer+= strlen($chunk);
+      return $chunk;
+    }
+  }
+
+  /** @return void */
+  public function close() {
+    if (null === $this->file || !$this->file->isOpen()) return;
+
+    $this->file->close();
+    $this->file->unlink();
+  }
+
+  /** Ensure the file (if any) is closed */
+  public function __destruct() {
+    $this->close();
+  }
+}
\ No newline at end of file
diff --git a/src/test/php/io/unittest/BufferTest.class.php b/src/test/php/io/unittest/BufferTest.class.php
new file mode 100755
index 000000000..6ce774141
--- /dev/null
+++ b/src/test/php/io/unittest/BufferTest.class.php
@@ -0,0 +1,108 @@
+<?php namespace io\unittest;
+
+use io\File;
+use io\streams\Buffer;
+use lang\{Environment, IllegalArgumentException, IllegalStateException};
+use test\{Assert, Before, Test, Values};
+
+class BufferTest {
+  const THRESHOLD= 128;
+  private $temp;
+
+  #[Before]
+  public function temp() {
+    $this->temp= Environment::tempDir();
+  }
+
+  #[Test]
+  public function can_create() {
+    new Buffer($this->temp, self::THRESHOLD);
+  }
+
+  #[Test]
+  public function threshold_must_be_larger_than_zero() {
+    Assert::throws(IllegalArgumentException::class, fn() => new Buffer($this->temp, -1));
+  }
+
+  #[Test, Values([1, 127, 128])]
+  public function uses_memory_under_threshold($length) {
+    $bytes= str_repeat('*', $length);
+
+    $fixture= new Buffer($this->temp, self::THRESHOLD);
+    $fixture->write($bytes);
+
+    Assert::null($fixture->file());
+    Assert::equals($length, $fixture->size());
+    Assert::equals($bytes, $fixture->read());
+  }
+
+  #[Test, Values([129, 256, 1024])]
+  public function stores_file_when_exceeding_threshold($length) {
+    $bytes= str_repeat('*', $length);
+
+    $fixture= new Buffer($this->temp, self::THRESHOLD);
+    $fixture->write($bytes);
+
+    Assert::instance(File::class, $fixture->file());
+    Assert::equals($length, $fixture->size());
+    Assert::equals($bytes, $fixture->read());
+  }
+
+  #[Test, Values([127, 128, 129])]
+  public function read_after_eof($length) {
+    $bytes= str_repeat('*', $length);
+
+    $fixture= new Buffer($this->temp, self::THRESHOLD);
+    $fixture->write($bytes);
+
+    Assert::equals($length, $fixture->available());
+    Assert::equals($bytes, $fixture->read());
+    Assert::equals(0, $fixture->available());
+    Assert::equals('', $fixture->read());
+  }
+
+  #[Test, Values([127, 128, 129])]
+  public function reset($length) {
+    $bytes= str_repeat('*', $length);
+
+    $fixture= new Buffer($this->temp, self::THRESHOLD);
+    $fixture->write($bytes);
+
+    Assert::equals($length, $fixture->available());
+    Assert::equals($bytes, $fixture->read());
+
+    $fixture->reset();
+
+    Assert::equals($length, $fixture->available());
+    Assert::equals($bytes, $fixture->read());
+  }
+
+  #[Test]
+  public function cannot_write_after_draining_started() {
+    $fixture= new Buffer($this->temp, self::THRESHOLD);
+    $fixture->write('Test');
+    Assert::false($fixture->draining());
+
+    $fixture->read();
+    Assert::true($fixture->draining());
+    Assert::throws(IllegalStateException::class, fn() => $fixture->write('Test'));
+  }
+
+  #[Test]
+  public function file_deleted_on_close() {
+    $fixture= new Buffer($this->temp, 0);
+    $fixture->write('Test');
+
+    $fixture->close();
+    Assert::false($fixture->file()->exists());
+  }
+
+  #[Test]
+  public function double_close() {
+    $fixture= new Buffer($this->temp, 0);
+    $fixture->write('Test');
+
+    $fixture->close();
+    $fixture->close();
+  }
+}
\ No newline at end of file