What are the practical limitations on string length in PHP?

Maximum Length of a String in PHP

Strings in PHP do not have a specific limit regarding their length in 64-bit builds as per PHP 7.0.0. However, 32-bit builds and earlier versions impose a limit of 2GB (2147483647 bytes).

In PHP 5.x, strings were restricted to 2³¹-1 bytes due to the use of a signed 32-bit integer to record the length.

Memory Limit

PHP scripts have a limitation on the total memory they can allocate for variables, effectively placing a cap on the length of a single string variable. This limit is set by the memory_limit directive in php.ini. It defaults to 128MB in PHP 5.2 and 8MB in earlier versions.

If no memory limit is specified, it defaults to a value set during compilation. Assigning a value of -1 in php.ini disables memory checks, allowing the script to use as much memory as the operating system provides.

Practical Limitations

The memory limit directive imposes a practical limitation on string length, regardless of the theoretical limit. Additionally, the size of the file can be a factor when slurping its contents using functions like file_get_contents().

Example

A simple demonstration:

<code class="php">ini_set('memory_limit', 1024*1024);
printf("memory: %d\n", memory_get_usage(true));

$str = str_repeat('a', 255*1024);
echo "Allocated string of 255KB\n";

printf("memory: %d\n", memory_get_usage(true));

$str = str_repeat('a', 256*1024);
echo "Allocated string of 256KB\n";
printf("memory: %d\n", memory_get_usage(true));</code>

Here, a memory limit of 1MB is set. Allocating 255KB of string memory does not exceed the limit, but attempting to allocate 256KB triggers a fatal error due to reaching the limit.

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