diff --git a/.gitignore b/.gitignore
index e938c8b..60d3ddf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,3 +7,4 @@ Tests/Resources/outputs/*
 !Tests/Resources/outputs/.gitkeep
 
 phpunit.xml
+.phpunit.result.cache
diff --git a/Command.php b/Command.php
index 91aa78b..fae8238 100644
--- a/Command.php
+++ b/Command.php
@@ -76,7 +76,7 @@ class Command
      */
     protected $version;
 
-    public function __construct(string $magickBinaryPath = null)
+    public function __construct(?string $magickBinaryPath = null)
     {
         $magickBinaryPath = self::findMagickBinaryPath($magickBinaryPath);
 
@@ -102,7 +102,7 @@ public function __construct(string $magickBinaryPath = null)
         $this->magickBinaryPath = $magickBinaryPath;
     }
 
-    public static function create(string $magickBinaryPath = null): self
+    public static function create(?string $magickBinaryPath = null): self
     {
         return new self($magickBinaryPath);
     }
@@ -148,7 +148,7 @@ private static function cleanPath(string $path, bool $rtrim = false): string
      *
      * @see https://imagemagick.org/script/command-line-tools.php
      */
-    public function getExecutable(string $binary = null): array
+    public function getExecutable(?string $binary = null): array
     {
         if (!\in_array($binary, static::ALLOWED_EXECUTABLES, true)) {
             throw new \InvalidArgumentException(\sprintf(
@@ -165,7 +165,7 @@ public function getExecutable(string $binary = null): array
     /**
      * Entirely reset all current command statements, and start a whole new command.
      */
-    public function newCommand(string $binary = null): self
+    public function newCommand(?string $binary = null): self
     {
         $this->env = [];
         $this->command = $binary ? $this->getExecutable($binary) : [];
@@ -185,7 +185,7 @@ public function convert(string $sourceFile, bool $checkIfFileExists = true): sel
     /**
      * @see https://imagemagick.org/script/mogrify.php
      */
-    public function mogrify(string $sourceFile = null): self
+    public function mogrify(?string $sourceFile = null): self
     {
         $this->newCommand('mogrify');
         if ($sourceFile) {
@@ -382,7 +382,6 @@ public function size($geometry): self
     /**
      * Create a colored canvas.
      *
-     *
      * @see http://www.imagemagick.org/Usage/canvas/
      */
     public function xc(string $canvasColor = 'none'): self
@@ -432,7 +431,7 @@ public function extent($geometry): self
     }
 
     /**
-     * @param string $gravity
+     * @param string|Gravity $gravity
      *
      * @see https://www.imagemagick.org/script/command-line-options.php#gravity
      */
diff --git a/README.md b/README.md
index 6fad9be..47e7d07 100644
--- a/README.md
+++ b/README.md
@@ -19,7 +19,7 @@ composer require orbitale/imagemagick-php
 Requirements
 ===============
 
-* PHP 7.1 or higher
+* PHP 7.2 or higher
 * [ImageMagick 7](https://www.imagemagick.org/) has to be installed on your server, and the binaries must be executable by the user running the PHP process.
 
 Settings
diff --git a/ReferenceClasses/Geometry.php b/ReferenceClasses/Geometry.php
index e1dbe1e..1839b95 100644
--- a/ReferenceClasses/Geometry.php
+++ b/ReferenceClasses/Geometry.php
@@ -39,11 +39,14 @@ class Geometry
      */
     private $value;
 
+    /**
+     * @param string|int|null $width
+     */
     public static function createFromParameters(
         $width = null,
-        int $height = null,
-        int $x = null,
-        int $y = null,
+        ?int $height = null,
+        ?int $x = null,
+        ?int $y = null,
         ?string $aspectRatio = self::RATIO_NONE
     ): string {
         $geometry = $width;
@@ -74,20 +77,21 @@ public static function createFromParameters(
                 $geometry .= ($y >= 0 ? '+' : '-').\abs($y);
             }
         } elseif (null !== $y) {
-            if (null !== $y) {
-                $geometry .= '+0'.($y >= 0 ? '+' : '-').\abs($y);
-            }
+            $geometry .= '+0'.($y >= 0 ? '+' : '-').\abs($y);
         }
 
         return $geometry;
     }
 
+    /**
+     * @param string|int|null $width
+     */
     public function __construct(
         $width = null,
-        int $height = null,
-        int $x = null,
-        int $y = null,
-        string $aspectRatio = self::RATIO_NONE
+        ?int $height = null,
+        ?int $x = null,
+        ?int $y = null,
+        ?string $aspectRatio = self::RATIO_NONE
     ) {
         $args = \func_get_args();
 
diff --git a/ReferenceClasses/Gravity.php b/ReferenceClasses/Gravity.php
index c5a7a16..0f91383 100644
--- a/ReferenceClasses/Gravity.php
+++ b/ReferenceClasses/Gravity.php
@@ -28,11 +28,13 @@ class Gravity
      */
     private $value;
 
-    public static function createFromParameters(string $gravity): string {
+    public static function createFromParameters(string $gravity): string
+    {
         return $gravity;
     }
 
-    public function __construct(string $gravity) {
+    public function __construct(string $gravity)
+    {
         $this->value = $gravity;
     }
 
diff --git a/References.php b/References.php
index 4a67f24..aeb59cf 100644
--- a/References.php
+++ b/References.php
@@ -248,6 +248,5 @@ public function threshold(string $threshold): string
             $threshold,
             'http://www.imagemagick.org/script/command-line-options.php#threshold'
         ));
-
     }
 }
diff --git a/Tests/CommandTest.php b/Tests/CommandTest.php
index 4300dd3..a67465e 100644
--- a/Tests/CommandTest.php
+++ b/Tests/CommandTest.php
@@ -34,7 +34,7 @@ public function testWrongConvertDirs($path, $expectedMessage, $expectedException
             $exception = $e->getMessage();
             $exceptionClass = \get_class($e);
         }
-        static::assertContains($expectedMessage, $exception);
+        static::assertStringContainsString($expectedMessage, $exception);
         static::assertEquals($expectedException, $exceptionClass);
     }
 
@@ -236,7 +236,7 @@ public function testConvertIdentifyImage($imageToIdentify, $expectedFormat, $exp
 
         $content = $response->getOutput();
 
-        static::assertContains(\sprintf(
+        static::assertStringContainsString(\sprintf(
             '%s %s %s %s %s',
             $imageToIdentify,
             $expectedFormat,
@@ -321,11 +321,10 @@ public function provideTestCommandString(): ?\Generator
         yield [$this->resourcesDir.'/moon_180.jpg', $this->resourcesDir.'/outputs/moon_geometry.jpg', '30x30+20+20', 50];
     }
 
-    /**
-     * @expectedException \InvalidArgumentException
-     */
     public function testWrongExecutable(): void
     {
+        $this->expectException(\InvalidArgumentException::class);
+
         $command = new Command(IMAGEMAGICK_DIR);
         $command->getExecutable('this_executable_might_not_exist');
     }
@@ -341,6 +340,6 @@ public function testInexistingFiles(): void
         } catch (\Exception $e) {
             $exception = $e->getMessage();
         }
-        static::assertContains(\sprintf('The file "%s" is not found.', $file), $exception);
+        static::assertStringContainsString(\sprintf('The file "%s" is not found.', $file), $exception);
     }
 }
diff --git a/Tests/References/ColorsTest.php b/Tests/References/ColorsTest.php
index 260f350..9e8c264 100644
--- a/Tests/References/ColorsTest.php
+++ b/Tests/References/ColorsTest.php
@@ -81,7 +81,7 @@ public function testIncorrectColors($color): void
         } catch (\InvalidArgumentException $e) {
             $msg = $e->getMessage();
         }
-        static::assertContains(
+        static::assertStringContainsString(
             \sprintf('The specified color (%s) is invalid', $color),
             $msg
         );
diff --git a/Tests/References/ColorspaceValuesTest.php b/Tests/References/ColorspaceValuesTest.php
index 7e7bf0f..360a728 100644
--- a/Tests/References/ColorspaceValuesTest.php
+++ b/Tests/References/ColorspaceValuesTest.php
@@ -66,7 +66,7 @@ public function testInvalidColorspaceValues($colorspaceValue): void
         } catch (\InvalidArgumentException $e) {
             $msg = $e->getMessage();
         }
-        static::assertContains(
+        static::assertStringContainsString(
             \sprintf('The specified colorspace value (%s) is invalid', \trim($colorspaceValue)),
             $msg
         );
diff --git a/Tests/References/InterlaceTypesTest.php b/Tests/References/InterlaceTypesTest.php
index 6416c45..3b63c52 100644
--- a/Tests/References/InterlaceTypesTest.php
+++ b/Tests/References/InterlaceTypesTest.php
@@ -70,7 +70,7 @@ public function testInvalidInterlaceTypes($interlaceType): void
         } catch (\InvalidArgumentException $e) {
             $msg = $e->getMessage();
         }
-        static::assertContains(
+        static::assertStringContainsString(
             \sprintf('The specified interlace type (%s) is invalid', \mb_strtolower(\trim($interlaceType))),
             $msg
         );
diff --git a/composer.json b/composer.json
index b49664b..c8fbd66 100644
--- a/composer.json
+++ b/composer.json
@@ -13,11 +13,12 @@
         }
     ],
     "require": {
-        "php": "^7.2",
+        "php": "^7.2|^8.0",
+        "ext-mbstring": "*",
         "symfony/process": "^4.0|^5.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.5"
+        "phpunit/phpunit": "^8.5|^9.5"
     },
     "minimum-stability": "stable",
     "autoload": {