You can use mb_strlen() to get the byte length using a encoding that only have byte-characters, without worring about multibyte or singlebyte strings. For example, as drake127 saids in a comment of mb_strlen, you can use '8bit' encoding:

<?php
    $string = 'Cién cañones por banda';
    echo mb_strlen($string, '8bit');
?>

You can have problems using strlen function since php have an option to overload strlen to actually call mb_strlen. See more info about it in http://php.net/manual/en/mbstring.overload.php

For trim the string by byte length without split in middle of a multibyte character you can use:

mb_strcut(string $str, int $start [, int $length [, string $encoding ]] )

Update for PHP 8.0 or great: Since PHP 8.0, Function Overloading was removed so you can always use strlen() to check the lenght in bytes.

http://php.net/manual/en/language.types.string.php says:

Note: As of PHP 7.0.0, there are no particular restrictions regarding the length of a string on 64-bit builds. On 32-bit builds and in earlier versions, a string can be as large as up to 2GB (2147483647 bytes maximum)

In PHP 5.x, strings were limited to 2³¹-1 bytes, because internal code recorded the length in a signed 32-bit integer.

You can slurp in the contents of an entire file, for instance using file_get_contents()

However, a PHP script has a limit on the total memory it can allocate for all variables in a given script execution, so this effectively places a limit on the length of a single string variable too.

This limit is the memory_limit directive in the php.ini configuration file. The memory limit defaults to 128MB in PHP 5.2, and 8MB in earlier releases.

If you don't specify a memory limit in your php.ini file, it uses the default, which is compiled into the PHP binary. In theory you can modify the source and rebuild PHP to change this default value.

If you specify -1 as the memory limit in your php.ini file, it stop checking and permits your script to use as much memory as the operating system will allocate. This is still a practical limit, but depends on system resources and architecture.

Re comment from @c2:

Here's a test:

<?php

// limit memory usage to 1MB 
ini_set('memory_limit', 1024*1024);

// initially, PHP seems to allocate 768KB for basic operation
printf("memory: %d\n",  memory_get_usage(true));

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

// now we have allocated all of the 1MB of memory allowed
printf("memory: %d\n",  memory_get_usage(true));

// going over the limit causes a fatal error, so no output follows
$str = str_repeat('a',  256*1024);
echo "Allocated string of 256KB\n";
printf("memory: %d\n",  memory_get_usage(true));