Learning lesson here: Never trust the PHP documentation. Test everything!

$position does not work as described, at least in PHP 7. After parsing without errors, it does not contain -1.

You can still use it to set the parsing start character position, but not necessarily whether any errors occurred.

To test for errors, replace my $nPosition test code in the other note with:
<?php 
// Check if errors
$bError = intl_is_failure(datefmt_get_error_code(oIDF));

// If no error
if(!$bError){
 // Use 'tm_hour' and 'tm_min' from the $aTime array for checking against other time values
 ...
}else{
 // Invalid string
 ...
}
?>

You can use datefmt_localtime to convert a user's time string, input using their locale's script, into standard numbers for checking, using a simple HTML input element.

This avoids using clunky HTML select elements formatted with hour and minute options in the locale's script to capture user input.

<?php
// Specify locale of user
$sLocale = 'en-us'; // US English user
$sLocale = 'ar-ye'; // Yemen Arab user

// Create a date formatter for the user's locale
$oIDF = datefmt_create($sLocale, IntlDateFormatter::NONE, IntlDateFormatter::SHORT);

// Simulate a user's time input string
$sTime = datefmt_format($oIDF, ['tm_hour'=>15, 'tm_min'=>25]);
// For a US English user, $sTime = '3:25 PM'
// For a Yemen Arab user, $sTime = '٣:٢٥ م'

// Specify datefmt_locale parsing start at the beginning of the user input
$nPosition = 0;

// Convert input time string to array of standard numeric values, as per localtime() output (http://php.net/manual/en/function.localtime.php)
$aTime = datefmt_localtime($oIDF, $sTime, $nPosition);
// If no errors, $nPosition set to -1 after parsing, else character position of error

// If valid string
if($nPosition = -1){
 // Use 'tm_hour' and 'tm_min' from the $aTime array for checking against other time values
 ...
}else{
 // Invalid string
 ...
}
?>