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).

Try this:

$array = array('this_that','is','an','array true');

function fixArrayKey(&$arr)
{
    $arr=array_combine(array_keys($arr),array_map(function($str){return str_replace(Array('true','_'),Array('YES',' '),$str);},array_values($arr)));
    foreach($arr as $key=>$val)
    {
        if(is_array($val)) fixArrayKey($arr[$key]);
    }
return $arr;
}

fixArrayKey($array);
print_r($array); 

echo implode("\n",$array);

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 )

foreach($all_regions as $key => $val) {
  $all_regions[$key] = strtolower(str_replace(' ', '_', $val));
}