Editing Troubleshooting of PHP errors (section)

=== 檔案操作有關 ===
可否讀取或寫入：例如 imagejpeg($canvas, $filename, 100); 沒有顯示錯誤訊息，則需要
* 檢查檔案或目錄是否存在 [http://tw2.php.net/file_exists file_exists]

* 檢查檔案是否可以讀取 [http://php.net/manual/en/function.is-readable.php is_readable]

* 如果是上傳檔案，需要檢查暫存目錄是否可以寫入 [http://stackoverflow.com/questions/1520956/php-way-to-find-the-web-servers-temp-path upload - PHP way to find the web server's temp path? - Stack Overflow]

* 檢查該檔案所在目錄是否已經建立，若未建立則需要建立 [http://php.net/manual/en/function.mkdir.php mkdir]，並設定可以寫入。如果產生位置是在多層子目錄下 (巢狀的多層目錄)，則需要啟用 recursive 選項<ref>[http://stackoverflow.com/questions/2303372/create-a-folder-if-it-doesnt-already-exist php - Create a folder if it doesn't already exist - Stack Overflow]  code snippet: <nowiki>mkdir('path/to/directory', 0755, true);</nowiki> </ref>

* 設定該檔案/目錄是否可以寫入: 如果要刪除或寫入檔案，需要檢查是否具備寫入權限
*# SELinux (Security-Enhanced Linux) policy: {{kbd | key = httpd_sys_rw_content_t}}<ref>[https://blog.lysender.com/2015/07/centos-7-selinux-php-apache-cannot-writeaccess-file-no-matter-what/ CentOS 7 + SELinux + PHP + Apache – cannot write/access file no matter what | Lysender's Daily Log Book]</ref>
*# (optional) 該檔案/目錄的 (1) 擁有者是網站使用者 (e.g. 網站伺服器設定檔預設值 CentOS 的 Apache 伺服器是 apache、Ubuntu 的 Apache 伺服器則是 www-data<ref>[https://ubuntuforums.org/showthread.php?t=1293508 [SOLVED] How to find out Apache Username?]</ref>) 、或者是 (2) 網站使用者的群組 (e.g. CentOS 的 Apache 伺服器是 apache、Nginx 伺服器是 nginx) 可以寫入
*# {{Win}} [http://www.webdeveloper.com/forum/showthread.php?165933-File-Permissions-in-Windows-(WAMP) File Permissions in Windows (WAMP)] <ref>[https://technet.microsoft.com/en-us/library/Cc754344.aspx Set, View, Change, or Remove Permissions on Files and Folders]</ref>

* 驗證是否具備寫入權限: 
*# 輸入 {{Linux}} 指令 (1) 檢查檔案權限 {{kbd | key = ls -Z /path/to/file}}、(2) 檢查目錄權限 {{kbd | key = ls -Zd /path/to/directory/}}
*# PHP [https://www.php.net/manual/en/function.get-current-user.php get_current_user] function 
*# PHP [http://php.net/manual/en/function.is-writable.php is_writable function]