feat: Add troubleshooting guide for permission denied errors during PHP installation

Author: pphatdev

docs/permission-denied.md

@@ -0,0 +1,76 @@
+# Permission Denied When Installing
+
+## Problem
+
+Installation fails with permission errors when trying to install a new PHP version.
+
+## Cause
+
+Installing PHP versions requires write access to the XAMPP directory (`C:\xampp`), which typically requires administrator privileges.
+
+## Solution
+
+Run PowerShell as Administrator before installing PHP versions.
+
+### How to Run PowerShell as Administrator
+
+**Method 1: Start Menu**
+1. Press the `Windows` key
+2. Type "PowerShell"
+3. Right-click on "Windows PowerShell"
+4. Select "Run as Administrator"
+
+**Method 2: Win + X Menu**
+1. Press `Win + X`
+2. Select "Windows PowerShell (Admin)" or "Terminal (Admin)"
+
+**Method 3: From File Explorer**
+1. Open File Explorer
+2. Navigate to any folder
+3. Click "File" → "Open Windows PowerShell" → "Open Windows PowerShell as administrator"
+
+### After Opening as Administrator
+
+Once you have an administrator PowerShell window, run your install command:
+
+```powershell
+phat install 8.2.27
+```
+
+## Verifying Administrator Access
+
+To check if you're running as administrator, look for "Administrator" in the PowerShell window title:
+
+```
+Administrator: Windows PowerShell
+```
+
+Or run this command:
+
+```powershell
+([Security.Principal.WindowsPrincipal] [Security.Principal.WindowsIdentity]::GetCurrent()).IsInRole([Security.Principal.WindowsBuiltInRole]::Administrator)
+```
+
+This should return `True` if you have administrator privileges.
+
+## Alternative: Run Specific Command as Admin
+
+If you don't want to open a new admin window, you can run a specific command with admin rights:
+
+1. Right-click on PowerShell/Terminal icon
+2. Select "Run as Administrator"
+3. In the admin prompt, run: `phat install [version]`
+
+## Common Permission Error Messages
+
+You might see errors like:
+- "Access to the path is denied"
+- "UnauthorizedAccessException"
+- "Administrator rights required"
+
+All of these indicate you need to run PowerShell as Administrator.
+
+## Related Resources
+
+- [Windows User Account Control](https://learn.microsoft.com/en-us/windows/security/application-security/application-control/user-account-control/)
+- [Back to Troubleshooting Index](TROUBLESHOOTING.md)