Get substring between two markers in a specific line of a file

I'm trying to select a specific section of a specific line within a file.

For instance, I want to isolate the first line of text from a file and get the numeric value of rate without the trailing kbit text.

Sample file contents:

tc class add dev br-lan parent 1:1 classid 1:2 htb rate 100kbit ceil 100kbit
tc class add dev br-lan parent 1:1 classid 1:3 htb rate 200kbit ceil 200kbit
tc class add dev br-lan parent 1:1 classid 1:4 htb rate 300kbit ceil 300kbit

I would like to select the data between "rate " (notice the space after rate) and "kbit" based on the line number, then store it as a variable.

Expected Result:

Effectively, I would like to retrieve the results based on, for example; line 1:

100

Then store this value as a variable.

This won't work with substr, as shown here: http://php.net/manual/en/function.substr.php as the length of the integer retrieved may not always be the same length.

File path:

../etc/init.d/bandwidthlimiters/rulestest

<?php

$output_arr=array();
$handle = fopen("inputfile.txt", "r");
if ($handle) {

    $i=1;
    while (($line = fgets($handle)) !== false) {

    $test_string=$line;
    $test_string=str_replace(" htb rate ","@@",$test_string);
    $test_string=str_replace("kbit ceil ","@@",$test_string);

    $tmp = explode("@@", $test_string);
    array_push($output_arr,$tmp[1]);
    $i++;
   }

    fclose($handle);
} else {
    // error opening the file.
} 

echo $output_arr[2]

?>