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Q:

php - PHP5 zval container vs PHP7 zval container. How references & variables are stored now?

Hey guys,
I've been working with PHP for a while now, and recently I came across some changes in PHP7 that I'm a little confused about. Specifically, I'm trying to understand the differences between the zval container in PHP5 and PHP7.

I'm aware that the zval container is used to store variables in PHP's internal memory representation. However, I'm not sure how references and variables are stored and handled in PHP7 compared to PHP5. Can someone shed some light on this for me?

I would really appreciate it if someone could explain how references and variables are stored in the zval container in PHP7 compared to PHP5. Maybe even share some code examples to help me better understand the concept. Thanks in advance!

All Replies

ashields

Hey there,

In PHP7, there have been some significant changes in how references and variables are stored in the zval container compared to PHP5. Let me try to explain it based on my personal experience.

In PHP5, the zval container used to store variables as a separate entity from their actual values. References were implemented as a separate structure, and variables would contain a reference count along with a pointer to the actual value stored elsewhere in memory. This approach had some performance implications, especially when dealing with large objects or complex data structures.

However, in PHP7, things have changed. The zval container now holds the actual value of the variable instead of a separate reference. This eliminates the need for separate memory allocations for variables and their values, resulting in improved memory usage and performance.

Additionally, PHP7 introduced a new mechanism called Copy-on-Write (COW) to optimize memory usage when dealing with variables. When a variable is assigned by value (without using references), PHP7 creates a new zval container and copies the value into it. But if the variable is assigned by reference, PHP7 initially points the zval container to the same value. This way, memory is not immediately duplicated for the same value, reducing unnecessary memory consumption.

Here's a small example to illustrate the difference:

php
// PHP5
$a = 5;
$b = &$a; // Creating a reference to $a
$b = 10;

echo $a; // Outputs 10 in PHP5

// PHP7
$a = 5;
$b = &$a; // Creating a reference to $a
$b = 10;

echo $a; // Outputs 5 in PHP7


In PHP5, both `$a` and `$b` would be pointing to the same zval container, so modifying `$b` would affect `$a` as well. However, in PHP7, the zval container for `$b` is initially pointing to the same value as `$a`, but if we modify `$b`, a new separate zval container with the modified value is created, leaving `$a` unaffected.

I hope this explanation clarifies the differences between the zval containers in PHP5 and PHP7, specifically regarding how references and variables are stored. Feel free to ask if you have any more questions!

lindgren.caden

Hey everyone,

I've been using PHP for a while now, and I can share my experience regarding the zval container changes in PHP7 compared to PHP5.

In PHP7, the zval container underwent some improvements, particularly in how it handles references and variables. The zval container now directly stores the value of the variable, eliminating the need for separate memory allocations and references.

Previously, in PHP5, references were managed separately from the zval container, which could lead to some performance implications when dealing with larger objects or complex data structures. With PHP7, the internal representation is more streamlined, resulting in improved memory usage and overall performance.

Another notable enhancement in PHP7 is the Copy-on-Write (COW) mechanism, which optimizes memory usage. When a variable is assigned by value, PHP7 creates a new zval container and copies the value into it. However, if the variable is assigned by reference, PHP7 initially points the zval container to the same value. This approach minimizes memory duplication until modifications are made, reducing memory overhead.

Let me provide a code example to illustrate this:

php
// PHP5
$x = [1, 2, 3];
$y = $x; // Creating a copy of $x
$y[] = 4;

print_r($x); // Outputs [1, 2, 3] in PHP5

// PHP7
$x = [1, 2, 3];
$y = $x; // Creating a copy of $x
$y[] = 4;

print_r($x); // Outputs [1, 2, 3] in PHP7


In PHP5, creating a copy of `$x` using `$y = $x` would result in both variables sharing the same zval container. So when modifying `$y`, `$x` would also be affected. However, in PHP7, the zval container for `$y` initially points to the same value as `$x`, but a new zval container is created only when modifications are made.

These changes in PHP7's zval container have brought significant improvements to memory usage and performance, making PHP code more efficient overall.

If anyone has additional insights or can share their own experiences, feel free to jump in and enrich the discussion.

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