From 0be8c639a97bf78109d229e711f59bb958fc7cb3 Mon Sep 17 00:00:00 2001
From: Jake Jackson <jake@gravitypdf.com>
Date: Mon, 7 Sep 2026 11:58:03 +1000
Subject: [PATCH] Tolerate empty OTL data when slicing a line of text
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applyOTL() resets $this->OTLdata to [] and returns early for a blank
string, but MultiCell() takes that empty array as "there is OTL data
here" and hands it to sliceOTLdata(), which reads 'group' and
'GPOSinfo' unconditionally. On PHP 8 that is two "Undefined array key"
warnings plus a null-to-substr() deprecation, for every MultiCell()
call with an empty or whitespace-only string.

Guard the two reads the way 'char_data' is already guarded, rather than
changing what applyOTL() leaves behind — ten call sites read
$this->otl->OTLdata and most test it for truthiness.

Mirrors mpdf/mpdf#2197 (mpdf/mpdf#2158).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
---
 src/Otl.php                    |  5 +--
 tests/Issues/Issue2158Test.php | 32 +++++++++++++++++
 tests/Mpdf/OtlTest.php         | 63 ++++++++++++++++++++++++++++++++++
 3 files changed, 98 insertions(+), 2 deletions(-)
 create mode 100644 tests/Issues/Issue2158Test.php
 create mode 100644 tests/Mpdf/OtlTest.php

diff --git a/src/Otl.php b/src/Otl.php
index 3a458afac..6e3176850 100644
--- a/src/Otl.php
+++ b/src/Otl.php
@@ -5747,10 +5747,11 @@ public function splitOTLdata(&$cOTLdata, $OTLcutoffpos, $OTLrestartpos = '')
 
 	public function sliceOTLdata($OTLdata, $pos, $len)
 	{
+		// applyOTL() leaves OTLdata empty for a blank string, so every key here is optional
 		$newOTLdata = ['GPOSinfo' => [], 'char_data' => []];
-		$newOTLdata['group'] = substr($OTLdata['group'], $pos, $len);
+		$newOTLdata['group'] = isset($OTLdata['group']) ? substr($OTLdata['group'], $pos, $len) : '';
 
-		if ($OTLdata['GPOSinfo']) {
+		if (!empty($OTLdata['GPOSinfo'])) {
 			foreach ($OTLdata['GPOSinfo'] as $k => $val) {
 				if ($k >= $pos && $k < ($pos + $len)) {
 					$newOTLdata['GPOSinfo'][($k - $pos)] = $val;
diff --git a/tests/Issues/Issue2158Test.php b/tests/Issues/Issue2158Test.php
new file mode 100644
index 000000000..0c53564aa
--- /dev/null
+++ b/tests/Issues/Issue2158Test.php
@@ -0,0 +1,32 @@
+<?php
+
+namespace Issues;
+
+class Issue2158Test extends \Yoast\PHPUnitPolyfills\TestCases\TestCase
+{
+
+	public function testMultiCellWithEmptyString()
+	{
+		$mpdf = new \Mpdf\Mpdf(['default_font' => 'dejavusans']);
+		$mpdf->AddPage();
+		$mpdf->MultiCell(0, 5, '');
+
+		$output = $mpdf->OutputBinaryData();
+		$this->assertStringStartsWith('%PDF-', $output);
+
+		$mpdf->cleanup();
+	}
+
+	public function testMultiCellWithWhitespaceOnlyString()
+	{
+		$mpdf = new \Mpdf\Mpdf(['default_font' => 'dejavusans']);
+		$mpdf->AddPage();
+		$mpdf->MultiCell(0, 5, "   \n   ");
+
+		$output = $mpdf->OutputBinaryData();
+		$this->assertStringStartsWith('%PDF-', $output);
+
+		$mpdf->cleanup();
+	}
+
+}
diff --git a/tests/Mpdf/OtlTest.php b/tests/Mpdf/OtlTest.php
new file mode 100644
index 000000000..3a2798f10
--- /dev/null
+++ b/tests/Mpdf/OtlTest.php
@@ -0,0 +1,63 @@
+<?php
+
+namespace Mpdf;
+
+use Mpdf\Fonts\FontCache;
+
+class OtlTest extends \Yoast\PHPUnitPolyfills\TestCases\TestCase
+{
+
+	/**
+	 * @var \Mpdf\Otl
+	 */
+	private $otl;
+
+	/**
+	 * @var \Mpdf\Mpdf
+	 */
+	private $mpdf;
+
+	protected function set_up()
+	{
+		parent::set_up();
+
+		$this->mpdf = new Mpdf(['mode' => 'c']);
+		$this->otl = new Otl($this->mpdf, new FontCache(new Cache(sys_get_temp_dir() . '/mpdf-otl-test')));
+	}
+
+	protected function tear_down()
+	{
+		parent::tear_down();
+
+		$this->mpdf->cleanup();
+	}
+
+	public function testSliceOfPopulatedData()
+	{
+		$OTLdata = [
+			'group' => 'SCCSC',
+			'GPOSinfo' => [1 => ['GPOSinfo'], 3 => ['other']],
+			'char_data' => [['bidi_class' => 0], ['bidi_class' => 1], ['bidi_class' => 2], ['bidi_class' => 3], ['bidi_class' => 4]],
+		];
+
+		$slice = $this->otl->sliceOTLdata($OTLdata, 1, 3);
+
+		$this->assertSame('CCS', $slice['group']);
+		$this->assertSame([0 => ['GPOSinfo'], 2 => ['other']], $slice['GPOSinfo']);
+		$this->assertCount(3, $slice['char_data']);
+	}
+
+	/**
+	 * applyOTL() resets OTLdata to an empty array for a blank string, and MultiCell() slices
+	 * whatever it is handed. See mpdf/mpdf#2158.
+	 */
+	public function testSliceOfEmptyDataReturnsAnEmptyStructure()
+	{
+		$slice = $this->otl->sliceOTLdata([], 0, 0);
+
+		$this->assertSame('', $slice['group']);
+		$this->assertSame([], $slice['GPOSinfo']);
+		$this->assertSame([], $slice['char_data']);
+	}
+
+}