another version of return_bytes which returns faster and does not use multiple multiplications (sorry:). even if it is resolved at compile time it is not a good practice;
no local variables are allocated;
the trim() is omitted (php already trimmed values when reading php.ini file);
strtolower() is replaced by second case which wins us one more function call for the price of doubling the number of cases to process (may slower the worst-case scenario when ariving to default: takes six comparisons instead of three comparisons and a function call);
cases are ordered by most frequent goes first (uppercase M-values being the default sizes);
specs say we must handle integer sizes so float values are converted to integers and 0.8G becomes 0;
'Gb', 'Mb', 'Kb' shorthand byte options are not implemented since are not in specs, see
http://www.php.net/manual/en/faq.using.php#faq.using.shorthandbytes

<?php
function return_bytes ($size_str)
{
    switch (substr ($size_str, -1))
    {
        case 'M': case 'm': return (int)$size_str * 1048576;
        case 'K': case 'k': return (int)$size_str * 1024;
        case 'G': case 'g': return (int)$size_str * 1073741824;
        default: return $size_str;
    }
}
?>

Here is another way to get the result in bytes using PHP8

<?php

    /**
     * @param string $size
     * @return int
     * @author DevsrealmGuy
     */
    public function getBytes(string $size): int
    {
        $size = trim($size);

        #
        # Separate the value from the metric(i.e MB, GB, KB)
        #
        preg_match('/([0-9]+)[\s]*([a-zA-Z]+)/', $size, $matches);

        $value = (isset($matches[1])) ? $matches[1] : 0;
        $metric = (isset($matches[2])) ? strtolower($matches[2]) : 'b';

        #
        # Result of $value multiplied by the matched case
        # Note: (1024 ** 2) is same as (1024 * 1024) or pow(1024, 2)
        #
        $value *= match ($metric) {
            'k', 'kb' => 1024,
            'm', 'mb' => (1024 ** 2),
            'g', 'gb' => (1024 ** 3),
            't', 'tb' =>  (1024 ** 4),
            default => 0
        };

        return (int)$value;
    }

        #
        # TEST: This default to 0 if it doesn't conform with the match standard
        #
        echo getBytes('2GB') . "</br>";
        # OUTPUT: 2147483648
        echo getBytes('4tb') . "</br>";
        # OUTPUT: 4398046511104
        echo getBytes('5345etrgrfd') . "</br>";
        # OUTPUT: 0
        echo getBytes('357568336586') . "</br>";
        # OUTPUT: 0
?>

Be aware that max_execution_time can be altered by XDebug.

While debugging a script locally that made use of <?php ini_get('max_execution_time'); ?> it returned 0 when XDebug remote debugging was enabled and the IDE was listening to it.

It makes sense, since debugging manually takes time so we don't want the script to time out ; but in that particular case, it made it look to the script like max_execution_time was 0, so calculations were wrong.

You can see in phpinfo() that local value is 0 in that case, but master value is the correct one you set in your php.ini.

Concerning the value retourned, it depends on how you set it.
I had the problem with horde-3 which test the safe_mode value.
THan :
- if you set the value with php_admin_value safe_mode Off (or On) ini_get returns the string
- if you set the value with php_admin_flag safe_mode Off (or On) ini_get returns the boolean.

This version of return_bytes takes care of the MB, GB, KB cases along with the M,G,K ones.
Hope this is helpful!

<?php
    public static function return_bytes ($val)
    {
        if(empty($val))return 0;

        $val = trim($val);

        preg_match('#([0-9]+)[\s]*([a-z]+)#i', $val, $matches);

        $last = '';
        if(isset($matches[2])){
            $last = $matches[2];
        }

        if(isset($matches[1])){
            $val = (int) $matches[1];
        }

        switch (strtolower($last))
        {
            case 'g':
            case 'gb':
                $val *= 1024;
            case 'm':
            case 'mb':
                $val *= 1024;
            case 'k':
            case 'kb':
                $val *= 1024;
        }

        return (int) $val;
    }
?>

Here is a version combining a few of the examples here that does *not* require php8 nor does it generate a warning

/**
* gets the value in bytes converted from a human readable string like 10G'
* 
* @param mixed $val the human readable/shorthand version of the value
* @return int the value converted to bytes
*/
function return_bytes($val) {
    $val = trim($val);
    preg_match('/([0-9]+)[\s]*([a-zA-Z]+)/', $val, $matches);
    $value = (isset($matches[1])) ? intval($matches[1]) : 0;
    $metric = (isset($matches[2])) ? strtolower($matches[2]) : 'b';
    switch ($metric) {
        case 'tb':
        case 't':
            $value *= 1024;
        case 'gb':
        case 'g':
            $value *= 1024;
        case 'mb':
        case 'm':
            $value *= 1024;
        case 'kb':
        case 'k':
            $value *= 1024;
    }
    return $value;
}

You can set custom entries in the ini file to provide globals such as database details.
However these must be retrieved with get_cfg_var, ini_get won't work.

Yet another implementation of return_bytes:

<?php
function return_bytes($val)
{
    assert('1 === preg_match("/^\d+([kmg])?$/i", $val)');
    static $map = array ('k' => 1024, 'm' => 1048576, 'g' => 1073741824);
    return (int)$val * @($map[strtolower(substr($val, -1))] ?: 1);
}
?>

If you're using PHP >= 7, you might replace ?: with ?? to avoid the use of the @ silencer.

The above example function called return_bytes() assumes that ini_get('upload_max_filesize') delivers only one letter at the end. As I've seen 'Mb' and things like that, I'd suggest to change the $last = ... part into $last = strtolower(substr($val,strlen($val/1),1)).
I'd call it $unit then.

Here is how to accurately test for boolean php.ini values:

<?php

function ini_get_bool($a)
{
    $b = ini_get($a);

    switch (strtolower($b))
    {
    case 'on':
    case 'yes':
    case 'true':
        return 'assert.active' !== $a;

    case 'stdout':
    case 'stderr':
        return 'display_errors' === $a;

    default:
        return (bool) (int) $b;
    }
}

?>

It might be useful for included scripts that include other files to extend the 'include_path' variable:

<?php ini_set('include_path',ini_get('include_path').':../includes:');  ?>

Sometimes, it may also be useful to store the current 'include_path' in a variable, overwrite it, include, and then restore the old 'include_path'.

In a similar vein, converting flags to booleans proper:

<?php
function return_boolean($val)
{
    static $map = array ('on' => true, 'true' => true, 'off' => false, 'false' => false);
    return @($map[strtolower($val)] ?: (bool)$val);
}
?>

If you're using PHP >= 7, consider replacing ?: with ?? and removing the @ silencer.