Check if all files within a range are in a directory.

index.php

<?php

global $argv;

$dir = __DIR__;

if ( isset( $argv[1] ) ) {
	$dir = $argv[1];
}

$numbers = range( '201800001', '201800620' );

if ( ! is_dir( $dir ) ) {
	var_dump( $dir );

	exit( 1 );
}

$files = scandir( $dir );

// @https://stackoverflow.com/a/10473026/6411283
function startsWith($haystack, $needle) {
    // search backwards starting from haystack length characters from the end
    return $needle === ''
      || strrpos($haystack, $needle, -strlen($haystack)) !== false;
}

foreach ( $numbers as $number ) {
	$number = strval( $number );

	foreach ( $files as $file ) {
		if ( startsWith( $file, $number ) ) {
			continue 2;
		}
	}

	echo $number, PHP_EOL;
}