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Developing software and web applications in PHP often involves solving errors that can arise during development. One common error that developers may encounter is "Fatal error: Call to undefined function". This article provides an expert look at the causes of this error and offers specific steps to resolve it.

Causes of the Error

The "Fatal error: Call to undefined function" error in PHP indicates that the script attempted to call a function that has not been defined. This issue can have several causes:

• The function has not yet been defined: If the function call is made before the function is defined in the code.
• Typo in the function name: A common mistake where a developer accidentally uses the wrong function name.
• Extension or library not properly installed: If the function belongs to an external library or PHP extension that has not been installed or enabled.
• Scope of visibility: The function may be defined in a different scope that is not accessible from the current place of call.

Resolving the Error

To deal with this error, a systematic approach is necessary. Here are the steps you should consider:

1. Check the function name: First, verify that the name of the function is spelled correctly. Also, check if you are using an obsolete function that has been removed from your version of PHP.

2. Ensure function definition before calling: Make sure that the function is defined before its first call in the code. This is especially important in PHP, where the order of the code is significant.

3. Check extensions and libraries: If the function belongs to an external library or extension, verify that this extension or library is properly installed and activated in your environment. This can be done using the phpinfo() function or the command line.

4. Verify scope of visibility: If the function is defined in a different file or within a class, ensure that this file is properly included (e.g., using require or include) or that the class is properly instantiated and the function is called statically if appropriate.

Tools and Techniques for Debugging

• Error reporting: Use error_reporting(E_ALL); and ini_set('display_errors', 1); at the beginning of the script to display all errors.
• Error logging: Set up error logging to a file for analyzing problems without displaying errors to users.
• Xdebug: Install and configure Xdebug for detailed error debugging and monitoring script behavior.

In conclusion, the "Fatal error: Call to undefined function" is an error that can be efficiently resolved by careful review and debugging of the code. Consistent testing and the use of modern development tools can help you quickly find and fix issues in your code.