[REF] Create polyfill function for array_key_first which only exists …

…in php7.3 and onwards

CRM/Contribute/Form/ContributionBase.php

@@ -1138,10 +1138,10 @@ public function cancelRecurring() {
    * Arguably the form should start to build $this->_params in the pre-process main page & use that array consistently throughout.
    */
   protected function setRecurringMembershipParams() {
-    $priceFieldId = array_key_first($this->_values['fee']);
+    $priceFieldId = CRM_Utils_Array::firstArrayKey($this->_values['fee']);
     // Why is this an array in CRM_Contribute_Form_Contribution_Main::submit and a string in CRM_Contribute_Form_Contribution_Confirm::preProcess()?
     if (is_array($this->_params["price_{$priceFieldId}"])) {
-      $priceFieldValue = array_key_first($this->_params["price_{$priceFieldId}"]);
+      $priceFieldValue = CRM_Utils_Array::firstArrayKey($this->_params["price_{$priceFieldId}"]);
     }
     else {
       $priceFieldValue = $this->_params["price_{$priceFieldId}"];

CRM/Utils/Array.php

@@ -1351,4 +1351,22 @@ public static function prefixKeys(array $collection, string $prefix) {
     return $result;
   }
 
+  /**
+   * Return the first Array Key either using array_key_first or a custom function
+   * @param array $array the array to look through
+   *
+   * @return string|int|null
+   */
+  public static function firstArrayKey(array $array) {
+    if (!function_exists('array_key_first')) {
+      foreach ($arr as $key => $unused) {
+        return $key;
+      }
+      return NULL;
+    }
+    else {
+      return array_key_first($array);
+    }
+  }
+
 }