If you just want to filter everything other than the numbers out, the easiest is to use filter_var:

$str = 'In My Cart : 11 items';
$int = (int) filter_var($str, FILTER_SANITIZE_NUMBER_INT);

You can make use of regular expressions to first match your pattern of characters (Case#) and then you expect to match numbers only (digits), that is \d in PCRE (Demo):

$numbers = preg_match("/Case#(\d+)/", $text, $matches)
              ? (int)$matches[1]
              : NULL
    ;
unset($matches);

For multiple (integer) matches at once:

$numbers = preg_match_all("/Case#(\d+)/", $text, $matches)
              ? array_map('intval', $matches[1])
              : NULL
    ;
unset($matches);

You can use regular expressions for this. The \d escape sequence will match all digits in the subject string.

For example:

<?php

function get_numerics ($str) {
    preg_match_all('/\d+/', matches);
    return $matches[0];
}

$one = 'foo bar 4 baz (5 qux quux)';
three = 'qux bar 12 quux (3 foo)';
$four = 'foo baz 3 bar (13 quux foo)';

print_r(get_numerics($one));
print_r(get_numerics($two));
print_r(get_numerics($three));
print_r(get_numerics($four));

https://3v4l.org/DiDBL

filter_var

You can use filter_var and sanitize the string to only include integers.

$s = "Lesson 001: Complete";
echo filter_var($s, FILTER_SANITIZE_NUMBER_INT);

https://eval.in/309989

preg_match

You can use a regular expression to match only integers.

$s = "Lesson 001: Complete";
preg_match("/([0-9]+)/", $s, $matches);
echo $matches[1];

https://eval.in/309994

Try splitting on whitespace (seem to be delimited by that)

$parts = preg_split('/\s+/', $input);
print $parts[1];