Description:
------------
The following script fills the memory with pointless data. Now the problem: From one point, the function "memory_get_usage" returns the same value, although the level has changed. This is important if you want to process data and save it only when a certain number of results have been generated. The second problem: Why are only 40% usable?

Test script:
---------------
<?php

    // C:\Users\User\Desktop\php-on-limit.cmd
    // @echo off
    // title PHP Development Server
    // cd "%cd%"
    // rem "C:\php\php.exe" "php-on-limit.php"
    // "C:\php\php.exe" "-d memory_limit=16M" "php-on-limit.php"
    // pause

    // C:\Users\User\Desktop\php-on-limit.php [PHP:7.3.3][PID:6336]

    //-------------------------------------------------
    // HEAD
    //-------------------------------------------------

    declare( strict_types = 1 );

    header( 'Content-Type:text/plain' );

    error_reporting( E_ALL );

    ini_set( 'display_errors', '1' );

    ini_set( 'html_errors', '0' );

    define( 'CORE_DIR', dirname( __FILE__ ) . DIRECTORY_SEPARATOR );

    isset( $argv ) or trigger_error( 'This is a command terminal application.', E_USER_ERROR );

    echo __FILE__ . ' [PHP:' . phpversion() . '][PID:' . getmypid() . ']' . PHP_EOL . PHP_EOL;

    //-------------------------------------------------
    // CODE
    //-------------------------------------------------

    $measuring_point_of_time = microtime( true );

    $memory_limit = conversion_back_to_bytes( ini_get( 'memory_limit' ) );

    echo 'memory_limit -- ' . human_readable_file_size( $memory_limit ) . ' -- defined limit' . PHP_EOL;

    // $memory_limit *= 0.48; // 48% of 128M can only be used

    // $memory_limit *= 0.47; // 47% of 64M can only be used

    $memory_limit *= 0.40; // 40% of 16M can only be used

    // If the 40% is exceeded, it comes to a fatal error, which is not desirable.

    // Fatal error: Allowed memory size of 16 MByte exhausted (tried to allocate 6 MByte).

    echo 'memory_limit -- ' . human_readable_file_size( $memory_limit ) . ' -- usable limit' . PHP_EOL;

    echo 'memory_allocated -- ' . human_readable_file_size( memory_get_usage( true ) ) . ' -- memory_used -- ' . memory_get_usage() . PHP_EOL;

    $something_has_to_be_calculated = true;

    $results_of_the_calculations = null;

    $last_memory_usage = memory_get_usage();

    while ( $something_has_to_be_calculated )
    {
        $results_of_the_calculations .= str_repeat( '1', 1024 );

        $currently_memory_usage = memory_get_usage();

        if ( $currently_memory_usage === $last_memory_usage )
        {
            echo PHP_EOL . 'Process recording: Memory function has not changed. currently_memory_usage and last_memory_usage are ' . $currently_memory_usage . PHP_EOL;

            break;
        }

        // echo chr( 0xD );

        echo 'memory_allocated -- ' . human_readable_file_size( memory_get_usage( true ) ) . ' -- memory_used -- ' . memory_get_usage() . ' -- data_length -- ' . strlen( $results_of_the_calculations );

        echo PHP_EOL;

        if ( memory_get_usage() >= 20 * 1024 * 1024 )
        {
            echo PHP_EOL . 'Process recording: Normal break.' . PHP_EOL;

            break;
        }

        if ( memory_get_usage() >= $memory_limit )
        {
            echo PHP_EOL . 'Process recording: Memory limit reached. Save calculated results so far.' . PHP_EOL;

            $results_of_the_calculations = null;
        }

        $last_memory_usage = memory_get_usage();
    }

    echo PHP_EOL;

    $results_of_the_calculations = null;

    echo 'Process recording: Calculations completed. Results are saved.' . PHP_EOL . PHP_EOL;

    echo 'Processing time: ' . elapsed_time( $measuring_point_of_time ) . ' seconds.' . PHP_EOL . PHP_EOL;

    //-------------------------------------------------
    // FUNCTIONS
    //-------------------------------------------------

    function elapsed_time( $measuring_point_of_time )
    {
        return number_format( microtime( true ) - $measuring_point_of_time, 3 );
    }

    function human_readable_file_size( $file_size, $precision = 2 )
    {
        $label = array( 'Bytes', 'kByte', 'MByte', 'GByte', 'TByte', 'PByte', 'EByte', 'ZByte', 'YByte' );

        return $file_size ? round( $file_size / pow( 1024, ( $index = floor( log( $file_size, 1024 ) ) ) ), $precision ) . ' ' . $label[ $index ] : '0 Bytes';
    }

    function conversion_back_to_bytes( $value )
    {
        $unit = strtolower( substr( $value, -1 ) );

        if ( $unit == 'k' ): return (integer) $value * 1024; endif;

        if ( $unit == 'm' ): return (integer) $value * 1048576; endif;

        if ( $unit == 'g' ): return (integer) $value * 1073741824; endif;

        return $value;
    }

?>

Expected result:
----------------
The "memory_get_usage" function must continually display the current memory usage to effectively operate at the memory limit.

Actual result:
--------------
C:\Users\User\Desktop\php-on-limit.php [PHP:7.3.4][PID:1224]

memory_limit -- 16 MByte -- defined limit
memory_limit -- 6.4 MByte -- usable limit
memory_allocated -- 2 MByte -- memory_used -- 410408
memory_allocated -- 2 MByte -- memory_used -- 411688 -- data_length -- 1024
memory_allocated -- 2 MByte -- memory_used -- 412968 -- data_length -- 2048
memory_allocated -- 2 MByte -- memory_used -- 414504 -- data_length -- 3072
memory_allocated -- 2 MByte -- memory_used -- 418600 -- data_length -- 4096

Process recording: Memory function has not changed. currently_memory_usage and last_memory_usage are 418520

Process recording: Calculations completed. Results are saved.

Processing time: 0.001 seconds.