feat: 增加var标签

extend/think/template/taglib/Cx.php

@@ -45,6 +45,7 @@ class Cx extends Taglib
         'notdefined' => ['attr' => 'name'],
         'load'       => ['attr' => 'file,href,type,value,basepath', 'close' => 0, 'alias' => ['import,css,js', 'type']],
         'assign'     => ['attr' => 'name,value', 'close' => 0],
+        'var'     => ['attr' => 'name,value', 'close' => 0],
         'define'     => ['attr' => 'name,value', 'close' => 0],
         'for'        => ['attr' => 'start,end,name,comparison,step'],
         'url'        => ['attr' => 'link,vars,suffix,domain', 'close' => 0, 'expression' => true],
@@ -577,6 +578,33 @@ class Cx extends Taglib
         return $parseStr;
     }
 
+    /**
+     * 如果变量在当前上下文不存在则设置，否则不设置
+     *
+     * @param array $tag
+     * @param string $content
+     * @return string
+     */
+    public function tagVar(array $tag, string $content): string
+    {
+        $name = $this->autoBuildVar($tag['name']);
+
+        if(!isset($tag['value'])) {
+            $value = 'null';
+        }else{
+            $flag = substr($tag['value'], 0, 1);
+    
+            if ('$' == $flag || ':' == $flag) {
+                $value = $this->autoBuildVar($tag['value']);
+            } else {
+                $value = '\'' . $tag['value'] . '\'';
+            }
+        }
+
+        $parseStr = '<?php if(!isset('.$name.')) { ' . $name . ' = ' . $value . ' ;} ?>';
+        return $parseStr;
+    }
+
     /**
      * define标签解析
      * 在模板中定义常量 支持变量赋值