替换字符串中的值 - PHP

I have a string 'some value' and I am looking for the function that replaces certain values with another one.

For example I need the

'some value'

to become

'some_value'

This is what I have tried so far with no luck

  $newCategory=str_replace(' ', '_', $rows["category"]); 
  echo $newCategory;

This works fine for me

$rows["category"] =  'some value';
$newCategory=str_replace(' ', '_', $rows["category"]); 
echo $newCategory;

Output

some_value

Display errors could be turned off in the php.ini or your apache config file.

You can turn it on in the script and see any error is there

 error_reporting(E_ALL);
 ini_set('display_errors', 1);

Your code works for me. Still you can give this a try:

$newCategory = preg_replace('/\s+/', '_', $rows["category"]);