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PHP: Only variables can be passed by reference

Hey everyone,

I'm currently working on a project using PHP, and I've encountered a situation where I need to pass a value by reference. However, I'm a bit confused because I keep getting the error message "Only variables can be passed by reference". I've done some research and it seems like this error is only related to passing variables by reference, but I'm not sure why that is the case.

To give you some context, I'm trying to pass a value to a function and modify it directly within the function. From what I understand, passing by reference allows me to modify the original value instead of creating a copy. But now I'm stuck because it seems like I can only pass variables by reference, and this limitation is causing some issues for me.

I've read the PHP documentation and searched online, but I couldn't find a clear explanation for why only variables can be passed by reference. Can someone please explain this to me? Why can't I pass other types of values, like constants or literals, by reference in PHP?

I appreciate any help or insights you can provide. Thank you!

All Replies


Hey there,

I've encountered this "Only variables can be passed by reference" error before, and I understand your confusion. So, let me try to shed some light on this based on my personal experience.

In PHP, passing by reference is a mechanism that allows you to pass a variable to a function or method and have any changes made to that variable reflected in the calling scope. However, this mechanism is restricted to variables only, and there are a couple of reasons for that.

Firstly, passing constants or literals by reference doesn't make sense because they are immutable, meaning their values cannot be modified once defined. When you attempt to pass a constant or literal by reference, PHP throws that error to prevent any unintended consequences and save system resources.

Secondly, passing objects or arrays by reference generally works as expected in PHP, but it's worth noting that objects and arrays are already passed by reference by default. So, explicitly passing them by reference isn't necessary in most cases, unless you specifically require the function to be able to modify the object or array itself.

To sum it up, the limitation of passing only variables by reference is there to ensure data consistency and prevent unintentional modifications to constants and literals. By sticking to passing variables, you can have more control over the reference behavior and avoid unexpected side effects.

I hope this explanation clarifies things for you. If you have any more questions, feel free to ask!


Hey there,

I completely understand your confusion regarding the error message "Only variables can be passed by reference"! It can be quite frustrating when you encounter this issue while working with PHP.

In my personal experience, I've come across this error when I was trying to pass a constant by reference in my PHP code. Initially, I couldn't understand why it wasn't allowed since I wanted the function to modify the constant's value directly. However, after some research and experimentation, I realized that PHP doesn't permit passing constants by reference for an important reason.

Constants, as the name suggests, are meant to hold values that remain fixed throughout the execution of a script. They are treated as read-only and cannot be changed once defined. So, if PHP permitted passing constants by reference, it would contradict the essence of constants and potentially lead to unexpected and unwanted modifications, violating the intended behavior.

By allowing only variables to be passed by reference, PHP ensures data integrity and prevents us from inadvertently altering values that we assumed to be constant. It's a protective measure to maintain the reliability and predictability of our code.

In scenarios where you need to modify a value within a function and have it reflected in the calling scope, considering using a variable instead of a constant. Variables can be passed by reference effortlessly, and you'll be able to achieve the desired behavior without any issues.

I hope this helps clarify the restriction on passing by reference in PHP and why constants cannot be passed in such a manner. Let me know if you have further questions or need more assistance!

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