Fix issue processwire/processwire-issues#901

wire/modules/Fieldtype/FieldtypeCheckbox.module

@@ -8,7 +8,7 @@
  * For documentation about the fields used in this class, please see:  
  * /wire/core/Fieldtype.php
  * 
- * ProcessWire 3.x, Copyright 2020 by Ryan Cramer
+ * ProcessWire 3.x, Copyright 2023 by Ryan Cramer
  * https://processwire.com
  *
  */
@@ -56,15 +56,22 @@ class FieldtypeCheckbox extends Fieldtype {
 	 * @param Field $field
 	 * @param int|null $value
 	 * @param string $property
-	 * @return MarkupFieldtype|string
+	 * @return string
 	 * 
 	 */
 	public function ___markupValue(Page $page, Field $field, $value = null, $property = '') {
 		if(is_null($value)) $value = $page->get($field->name); 
 		$checked = $value ? " checked='checked'" : "";
-		$textValue = $value ? '☒': '☐'; // utf-8 checkbox (the ☑ is also works for checked, but not quite as visible)
+		if($this->wire()->page->template->name === 'admin') {
+			// use fa-icon for checkbox in the admin
+			$out = $checked ? wireIconMarkup('check-square-o') : wireIconMarkup('square-o');
+		} else {
+			// use disabled checkbox outside of the admin
+			$out = "<input type='checkbox' $checked disabled='disabled' />";
+		}
 		// note: the span below ensures a value survives even if markupValue has tags stripped from it
-		return "<input type='checkbox' $checked disabled='disabled' /><span style='display:none'>$textValue</span>";
+		$textValue = $checked ? '☒' : '☐'; // utf-8 checkbox (the ☑ also works for checked, but not quite as visible)
+		return $out . "<span style='display:none'>$textValue</span>";
 	}
 
 	/**
@@ -148,7 +155,6 @@ class FieldtypeCheckbox extends Fieldtype {
 	 *
 	 */
 	public function ___getCompatibleFieldtypes(Field $field) {
-		if($field) {}
 		$fieldtypes = $this->wire(new Fieldtypes());
 		$toggleClass = 'FieldtypeToggle';
 		$allowToggle = $this->wire()->modules->isInstalled($toggleClass);
@@ -165,4 +171,3 @@ class FieldtypeCheckbox extends Fieldtype {
 	}
 
 }
-