Updated composer.json, tested and works with PHP 7.4

composer.json

@@ -15,6 +15,11 @@
         }
     ],
     "bin": [ "bin/dataset-info" ],
+    "require": {
+        "php": "^7.4|^8.0",
+        "ext-simplexml": "*",
+        "ext-json": "*"
+    },
     "require-dev": {
         "phpunit/phpunit": "^9.5",
         "phpstan/phpstan": "^1.8"

src/Readers/CsvReader.php

@@ -70,13 +70,13 @@ class CsvReader implements ReaderInterface
         $this->checkLoadedSlice();
     }
 
-    public function key(): mixed
+    public function key()
     {
         //$this->checkLoadedSlice();
         return $this->counter;
     }
 
-    public function current(): mixed
+    public function current()
     {
         //$this->checkLoadedSlice();   
         return $this->data[$this->currentIndex];
@@ -102,4 +102,4 @@ class CsvReader implements ReaderInterface
         return ($this->currentFile < count($this->files) && ($this->currentIndex < count($this->data)));
         
     }
-}
+}

src/Readers/JsonReader.php

@@ -61,13 +61,13 @@ class JsonReader implements ReaderInterface
         $this->checkLoadedSlice();
     }
 
-    public function key(): mixed
+    public function key()
     {
         //$this->checkLoadedSlice();
         return $this->counter;
     }
 
-    public function current(): mixed
+    public function current()
     {
         //$this->checkLoadedSlice();   
         return $this->data[$this->currentIndex];