get byte value from shorthand byte notation in php.ini

Is there any way to get the byte values from the strings returned from functions like ini_get('upload_max_filesize') and ini_get('post_max_size') when they are using shorthand byte notation? For example get 4194304 from 4M ? I could hack together a function that does this but I would be surprised if there wasn't some built in way of doing this.

The paragraph you linked to ends:

You may not use these shorthand notations outside of php.ini, instead use an integer value of bytes. See the ini_get() documentation for an example on how to convert these values.

This leads you to something like this (which I have slightly modified):

function return_bytes($val)
{
    $val  = trim($val);

    if (is_numeric($val))
        return $val;

    $last = strtolower($val[strlen($val)-1]);
    $val  = substr($val, 0, -1); // necessary since PHP 7.1; otherwise optional

    switch($last) {
        // The 'G' modifier is available since PHP 5.1.0
        case 'g':
            $val *= 1024;
        case 'm':
            $val *= 1024;
        case 'k':
            $val *= 1024;
    }

    return $val;
}

Use it like so:

echo return_bytes("3M");
// Output: 3145728

There is no built-in function to perform this task; recall that, really, INI settings are designed for use internally within PHP. The PHP source uses a similar function to the above.