为什么在脚本中找不到PHP类在另一个脚本中工作正常？

Question

I have two scripts. Both scripts are located in the same folder on the same server.

Script 1:

<?php

$text = "Is this not working? Why, yes it is.";

$locale = 'en_US';

$i = IntlBreakIterator::createSentenceInstance($locale);
$i->setText($text);

foreach($i->getPartsIterator() as $sentence) {
    echo $sentence . PHP_EOL . '----- next -----' .  PHP_EOL;
}

?>

Script 2:

... bunch of other code ...

$doc = file_get_contents("file.txt");

$locale = 'en_US';

$i = IntlBreakIterator::createSentenceInstance($locale);
$i->setText($doc);

foreach($i->getPartsIterator() as $sentence) {
    echo $sentence . PHP_EOL . '----- next -----' .  PHP_EOL;
}

... bunch of other code ...

Script 1 executes as expected, with the desired output and no errors. Script 2 does not execute and PHP keeps throwing the error PHP Fatal error: Uncaught Error: Class 'IntlBreakIterator' not found

What is going on here?