As of others have explained how to do it using str_replace, you can also use regex to achieve this.

$name = preg_replace('/\s+/', '_', $name);

Try this instead:

$journalName = preg_replace('/\s+/', '_', $journalName);

Explanation: you are most likely seeing whitespace, not just plain spaces (there is a difference).

str_replace

Need to use array_map function like as

$state = array("gujarat","andhra_pradesh","madhya_pradesh","uttar_pradesh");
$state = array_map(upper, $state);
function upper($state){
    return str_replace('_', ' ', ucwords($state));
}
print_r($state);// output Array ( [0] => Gujarat [1] => Andhra pradesh [2] => Madhya pradesh [3] => Uttar pradesh )

Replacing spaces is fine, but I might suggest going a little further to handle other URL-hostile characters like question marks, apostrophes, exclamation points, etc.

Also note that the general consensus among SEO experts is that dashes are preferred to underscores in URLs.

import re

def urlify(s):

    # Remove all non-word characters (everything except numbers and letters)
    s = re.sub(r"[^\w\s]", '', s)

    # Replace all runs of whitespace with a single dash
    s = re.sub(r"\s+", '-', s)

    return s

# Prints: I-cant-get-no-satisfaction"
print(urlify("I can't get no satisfaction!"))