preserve compatability with PHPUnit < 8.0 which do not support the assertString(Not)ContainsString assertion

Author: jakobbuis

src/MarkupAssertionsTrait.php

@@ -114,11 +114,19 @@ public function assertNotHasElementWithAttributes($attributes = [], $output = ''
      */
     public function assertElementContains($contents, $selector = '', $output = '', $message = '')
     {
-        $this->assertStringContainsString(
-            $contents,
-            $this->getInnerHtmlOfMatchedElements($output, $selector),
-            $message
-        );
+        if (method_exists($this, 'assertStringContainsString')) {
+            $this->assertStringContainsString(
+                $contents,
+                $this->getInnerHtmlOfMatchedElements($output, $selector),
+                $message
+            );
+        } else {
+            $this->assertContains(
+                $contents,
+                $this->getInnerHtmlOfMatchedElements($output, $selector),
+                $message
+            );
+        }
     }
 
     /**
@@ -133,11 +141,20 @@ public function assertElementContains($contents, $selector = '', $output = '', $
      */
     public function assertElementNotContains($contents, $selector = '', $output = '', $message = '')
     {
-        $this->assertStringNotContainsString(
-            $contents,
-            $this->getInnerHtmlOfMatchedElements($output, $selector),
-            $message
-        );
+        if (method_exists($this, 'assertStringContainsString')) {
+            $this->assertStringNotContainsString(
+                $contents,
+                $this->getInnerHtmlOfMatchedElements($output, $selector),
+                $message
+            );
+        }
+        else {
+            $this->assertNotContains(
+                $contents,
+                $this->getInnerHtmlOfMatchedElements($output, $selector),
+                $message
+            );
+        }
     }
 
     /**