Coding⏱️ 3 min read📅 2026-06-11

How to Fix: Error compiling and installing mcrypt from source

Error compiling and installing mcrypt from source on RHEL-6 with PHP5.4.

Quick Answer: The issue is caused by the missing ac_nonexistent.h file, which is required for the C preprocessor. You need to install the libglib2-devel package, which includes this header file, and then re-run the configure command.

📋 Table of Contents

  1. 🛑 Root Causes of the Error
  2. ✅ Best Solutions to Fix It
  3. ✨ Wrapping Up

The error 'configure: WARNING: mcrypt.h: accepted by the compiler, rejected by the preprocessor!' occurs when the compiler accepts the mcrypt header file but the preprocessor is unable to find it. This issue affects users who are trying to compile PHP5.4 with the mcrypt extension enabled from source.

This error can be frustrating for developers as it prevents them from successfully installing and compiling the mcrypt extension, which is essential for certain PHP features.

🛑 Root Causes of the Error

  • The primary reason for this error is that the preprocessor is unable to find the ac_nonexistent.h file, which is a header file required by the mcrypt library. This can happen if the development environment is not properly set up or if the necessary libraries are not installed.
  • An alternative reason could be that the compiler is accepting the mcrypt header file but the preprocessor is rejecting it due to compatibility issues with the system's C standard.

✅ Best Solutions to Fix It

Setting up the Development Environment

  1. Step 1: Step 1: Check if the necessary libraries are installed and properly configured. Ensure that the development environment is set up correctly, including the installation of all required dependencies.
  2. Step 2: Step 2: Verify that the LD_LIBRARY_PATH and LDFLAGS environment variables are correctly set to point to the locations of the mcrypt and mhash libraries.
  3. Step 3: Step 3: Re-run the configure command with the correct environment variables exported.
  4. Step 4: method_2_name
  5. Step 5: Alternative Fix Method: Compiling with Additional Flags
  6. Step 6: method_2_steps
  7. Step 7: ['Step 1: Add the -I flag to the CFLAGS variable to tell the compiler to look for header files in the specified directories.', 'Step 2: Re-run the configure command with the updated CFLAGS variable.', 'conclusion']
  8. Step 8: Additional Tips and Considerations
  9. Step 9: When compiling PHP5.4 with the mcrypt extension enabled, it's essential to ensure that all necessary libraries are installed and properly configured. Additionally, verifying the LD_LIBRARY_PATH and LDFLAGS environment variables can help resolve this issue. If the problem persists, consider consulting the official PHP documentation or seeking assistance from a Linux expert.

Alternative Advanced Fix

    ✨ Wrapping Up

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