Learn Handling Large Arrays, Generators, and Unset | Memory Profiling and Optimization

When working with large datasets in PHP, memory management becomes a critical concern. Arrays are a common way to store and process data, but they can consume significant memory, especially as the dataset grows. Each element in a PHP array is actually a zval structure, and PHP arrays are hash tables under the hood, which adds memory overhead per element. This means that loading a huge dataset into an array can quickly exhaust available memory and lead to performance issues or script termination due to memory limits.

Generators provide a more memory-efficient alternative for processing large datasets. Introduced in PHP 5.5, generators allow you to iterate over data without creating and storing the entire dataset in memory at once. Instead, a generator yields one value at a time, pausing execution between yields and resuming when the next value is requested. This approach dramatically reduces memory usage, as only one item is held in memory at any given time.

Another important technique for optimizing memory usage is the use of the unset function. By unsetting variables or array elements that are no longer needed, you signal to the PHP engine that their memory can be reclaimed. This is especially useful when processing large arrays in chunks or after extracting the necessary data from a large structure. However, keep in mind that unsetting a variable only marks it for garbage collection; the actual memory may not be immediately released if there are still references to the variable.

Combining these strategies—choosing the right data structures, using generators for sequential data processing, and unsetting variables promptly—can help you write PHP scripts that handle large datasets efficiently and avoid memory exhaustion.

memory_comparison.php


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<?php
// Compare memory usage: arrays vs generators, and demonstrate unset

// Helper function to report memory usage in MB
function reportMemory($message) {
    echo $message . ': ' . round(memory_get_usage() / 1024 / 1024, 2) . " MB\n";
}

// 1. Using a large array
reportMemory("Before creating array");

$largeArray = [];
for ($i = 0; $i < 1000000; $i++) {
    $largeArray[] = $i;
}

reportMemory("After creating large array");

// Process the array (dummy operation)
$sum = 0;
foreach ($largeArray as $value) {
    $sum += $value;
}

reportMemory("After processing large array");

// Free memory using unset
unset($largeArray);
reportMemory("After unset array");

// 2. Using a generator
function numberGenerator($max) {
    for ($i = 0; $i < $max; $i++) {
        yield $i;
    }
}

reportMemory("Before generator processing");

$sumGen = 0;
foreach (numberGenerator(1000000) as $value) {
    $sumGen += $value;
}

reportMemory("After generator processing");

/*
Output will show that:
- Creating a large array increases memory usage significantly.
- Unsetting the array frees memory (as seen in the next memory report).
- Using a generator keeps memory usage low, as only one value is in memory at a time.
*/

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Section 3. Chapter 3

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memory_comparison.php


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<?php
// Compare memory usage: arrays vs generators, and demonstrate unset

// Helper function to report memory usage in MB
function reportMemory($message) {
    echo $message . ': ' . round(memory_get_usage() / 1024 / 1024, 2) . " MB\n";
}

// 1. Using a large array
reportMemory("Before creating array");

$largeArray = [];
for ($i = 0; $i < 1000000; $i++) {
    $largeArray[] = $i;
}

reportMemory("After creating large array");

// Process the array (dummy operation)
$sum = 0;
foreach ($largeArray as $value) {
    $sum += $value;
}

reportMemory("After processing large array");

// Free memory using unset
unset($largeArray);
reportMemory("After unset array");

// 2. Using a generator
function numberGenerator($max) {
    for ($i = 0; $i < $max; $i++) {
        yield $i;
    }
}

reportMemory("Before generator processing");

$sumGen = 0;
foreach (numberGenerator(1000000) as $value) {
    $sumGen += $value;
}

reportMemory("After generator processing");

/*
Output will show that:
- Creating a large array increases memory usage significantly.
- Unsetting the array frees memory (as seen in the next memory report).
- Using a generator keeps memory usage low, as only one value is in memory at a time.
*/

Everything was clear?

Thanks for your feedback!

Section 3. Chapter 3