$name = str_replace(' ', '_', $name);

I'll suggest that you use this as it will check for both single and multiple occurrence of white space (as suggested by Lucas Green).

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

instead of:

$journalName = str_replace(' ', '_', $journalName);

use the str_replace() function:

<?php
    $old = 'OMG_This_Is_A_One_Stupid_Error';
    $new = str_replace(' ', '_', $old);
    echo $old; // will output OMG This Is A One Stupid error
?>

Reverse the parameters to obtain the reverse effect

<?php
    $old = 'OMG This Is A One Stupid_Error';
    $new = str_replace('_', ' ', $old);
    echo $old; // will output OMG_This_Is_A_One_Stupid error
?>

You can use the array_map function.

function modify($str) {
    return ucwords(str_replace("_", " ", $str));
}

Then in just use the above function as follows:

$states=array_map("modify", $old_states)

How about?

$before = '<product name_here>Product A</product_name here>';
$after = preg_replace('/(<[^>]*)\s+([^<]*>)/', '$1_$2', $before);
echo $after;

This should give

<product_name_here>Product A</product_name_here>

The parts before and after \s+ specify that you don't want the spaces outside of a tag pairing but just the ones who are enclosed between an opening tag < and a closing tag >. The $1 and $2 substitute back in the strings before and after the replaced whitespace.