diff --git a/composer.json b/composer.json
index de3afb1e55..3678c8eb62 100644
--- a/composer.json
+++ b/composer.json
@@ -92,7 +92,7 @@
         "phpcompatibility/php-compatibility": "^9.3",
         "phpstan/phpstan": "^1.1",
         "phpstan/phpstan-phpunit": "^1.0",
-        "phpunit/phpunit": "^8.5 || ^9.0",
+        "phpunit/phpunit": "^8.5 || ^9.0 || ^10.0",
         "squizlabs/php_codesniffer": "^3.7",
         "tecnickcom/tcpdf": "^6.5"
     },
diff --git a/src/PhpSpreadsheet/Helper/Handler.php b/src/PhpSpreadsheet/Helper/Handler.php
new file mode 100644
index 0000000000..d05197cef0
--- /dev/null
+++ b/src/PhpSpreadsheet/Helper/Handler.php
@@ -0,0 +1,46 @@
+<?php
+
+namespace PhpOffice\PhpSpreadsheet\Helper;
+
+class Handler
+{
+    /** @var string */
+    private static $invalidHex = 'Y';
+
+    // A bunch of methods to show that we continue
+    // to capture messages even using PhpUnit 10.
+    public static function suppressed(): bool
+    {
+        return @trigger_error('hello');
+    }
+
+    public static function deprecated(): string
+    {
+        return (string) hexdec(self::$invalidHex);
+    }
+
+    public static function notice(string $value): void
+    {
+        date_default_timezone_set($value);
+    }
+
+    public static function warning(): bool
+    {
+        return file_get_contents(__FILE__ . 'noexist') !== false;
+    }
+
+    public static function userDeprecated(): bool
+    {
+        return trigger_error('hello', E_USER_DEPRECATED);
+    }
+
+    public static function userNotice(): bool
+    {
+        return trigger_error('userNotice', E_USER_NOTICE);
+    }
+
+    public static function userWarning(): bool
+    {
+        return trigger_error('userWarning', E_USER_WARNING);
+    }
+}
diff --git a/src/PhpSpreadsheet/Shared/OLE/ChainedBlockStream.php b/src/PhpSpreadsheet/Shared/OLE/ChainedBlockStream.php
index c61ee25946..bce5fe34a4 100644
--- a/src/PhpSpreadsheet/Shared/OLE/ChainedBlockStream.php
+++ b/src/PhpSpreadsheet/Shared/OLE/ChainedBlockStream.php
@@ -6,6 +6,9 @@
 
 class ChainedBlockStream
 {
+    /** @var mixed */
+    public $context;
+
     /**
      * The OLE container of the file that is being read.
      *
diff --git a/tests/PhpSpreadsheetTests/Chart/Issue589Test.php b/tests/PhpSpreadsheetTests/Chart/Issue589Test.php
index 747cba7485..68747f6c58 100644
--- a/tests/PhpSpreadsheetTests/Chart/Issue589Test.php
+++ b/tests/PhpSpreadsheetTests/Chart/Issue589Test.php
@@ -108,7 +108,7 @@ public function testLineChartFill(): void
                     if ($actualXml === false) {
                         self::fail('Failure saving the spPr element as xml string!');
                     } else {
-                        self::assertXmlStringEqualsXmlString('<c:spPr><a:ln><a:solidFill><a:srgbClr val="98B954"/></a:solidFill></a:ln></c:spPr>', $actualXml);
+                        self::assertSame('<c:spPr><a:ln><a:solidFill><a:srgbClr val="98B954"/></a:solidFill></a:ln></c:spPr>', $actualXml);
                     }
                 }
             }
@@ -153,7 +153,7 @@ public function testLineChartFillIgnoresColorArray(): void
                     if ($actualXml === false) {
                         self::fail('Failure saving the spPr element as xml string!');
                     } else {
-                        self::assertXmlStringEqualsXmlString('<c:spPr><a:ln/></c:spPr>', $actualXml);
+                        self::assertSame('<c:spPr><a:ln/></c:spPr>', $actualXml);
                     }
                 }
             }
diff --git a/tests/PhpSpreadsheetTests/Helper/HandlerTest.php b/tests/PhpSpreadsheetTests/Helper/HandlerTest.php
new file mode 100644
index 0000000000..b0c432d1b9
--- /dev/null
+++ b/tests/PhpSpreadsheetTests/Helper/HandlerTest.php
@@ -0,0 +1,83 @@
+<?php
+
+namespace PhpOffice\PhpSpreadsheetTests\Helper;
+
+use PhpOffice\PhpSpreadsheet\Helper\Handler;
+use PHPUnit\Framework\TestCase;
+use Throwable;
+
+class HandlerTest extends TestCase
+{
+    public function testSuppressed(): void
+    {
+        self::assertTrue(Handler::suppressed());
+    }
+
+    public function testDeprecated(): void
+    {
+        if (method_exists(\PHPUnit\Framework\TestCase::class, 'setOutputCallback')) {
+            self::assertTrue(true);
+        } else {
+            try {
+                Handler::deprecated();
+                self::fail('Expected error/exception did not happen');
+            } catch (Throwable $e) {
+                self::assertStringContainsString('Invalid characters', $e->getMessage());
+            }
+        }
+    }
+
+    public function testNotice(): void
+    {
+        try {
+            Handler::notice('invalidtz');
+            self::fail('Expected error/exception did not happen');
+        } catch (Throwable $e) {
+            self::assertStringContainsString('Timezone', $e->getMessage());
+        }
+    }
+
+    public function testWarning(): void
+    {
+        try {
+            Handler::warning();
+            self::fail('Expected error/exception did not happen');
+        } catch (Throwable $e) {
+            self::assertStringContainsString('ailed to open stream', $e->getMessage());
+        }
+    }
+
+    public function testUserDeprecated(): void
+    {
+        if (method_exists(\PHPUnit\Framework\TestCase::class, 'setOutputCallback')) {
+            self::assertTrue(true);
+        } else {
+            try {
+                Handler::userDeprecated();
+                self::fail('Expected error/exception did not happen');
+            } catch (Throwable $e) {
+                self::assertStringContainsString('hello', $e->getMessage());
+            }
+        }
+    }
+
+    public function testUserNotice(): void
+    {
+        try {
+            Handler::userNotice();
+            self::fail('Expected error/exception did not happen');
+        } catch (Throwable $e) {
+            self::assertStringContainsString('userNotice', $e->getMessage());
+        }
+    }
+
+    public function testUserWarning(): void
+    {
+        try {
+            Handler::userWarning();
+            self::fail('Expected error/exception did not happen');
+        } catch (Throwable $e) {
+            self::assertStringContainsString('userWarning', $e->getMessage());
+        }
+    }
+}
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index 9ebd3f2623..fef59a5a7e 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -4,3 +4,34 @@
 
 // PHP 5.3 Compat
 //date_default_timezone_set('Europe/London');
+
+function phpunit10ErrorHandler(int $errno, string $errstr, string $filename, int $lineno): bool
+{
+    $x = error_reporting() & $errno;
+    if (
+        in_array(
+            $errno,
+            [
+                E_DEPRECATED,
+                E_WARNING,
+                E_NOTICE,
+                E_USER_DEPRECATED,
+                E_USER_NOTICE,
+                E_USER_WARNING,
+            ],
+            true
+        )
+    ) {
+        if (0 === $x) {
+            return true; // message suppressed - stop error handling
+        }
+
+        throw new \Exception("$errstr $filename $lineno");
+    }
+
+    return false; // continue error handling
+}
+
+if (!method_exists(\PHPUnit\Framework\TestCase::class, 'setOutputCallback')) {
+    set_error_handler('phpunit10ErrorHandler');
+}