PHP：简单的数学函数返回null而不是整数

I got a function:
- which should add to number $x random number from -25 to 25
- if result is higher than 255 or lower than 0, the function should be called again to generate acceptable result.

What I got is:

function changeParam($base_val)
{

    $plus = mt_rand(-25, 25);

    if (($base_val + $plus) > 255 || ($base_val + $plus) < 0) 
    {
        changeParam($base_val); //is it correct?
    } 
    else 
    {   
        $new_val = $base_val + $plus;
        return $new_val;        
    }   

}

Why this:

$x = changeParam(255);
var_dump($x);

does give me null sometimes?

I've tried to check it, but I didn't find the reason:

function changeParam($base_val)
{

    $plus = mt_rand(-25, 25);

    if (($base_val + $plus) > 255 || ($base_val + $plus) < 0) 
    {
        changeParam($base_val);
    } 
    else 
    {   
        $new_val = $base_val + $plus;
        echo 'Is there a problem? ' . $new_val;   // this line shows correct new_val
        var_dump($new_val);                       // correct, for instance: 'int 250'
        return $new_val;                           // but in the same time, the result = 'null'
    }   

}

If you really want to go with recursion, you have to add a return before the call to changeParam($base_val);. So it would look like this:

function changeParam($base_val)
{

    $plus = mt_rand(-25, 25);

    if (($base_val + $plus) > 255 || ($base_val + $plus) < 0) 
    {
        return changeParam($base_val); // CHANGED HERE
    } 
    else 
    {   
        $new_val = $base_val + $plus;
        return $new_val;        
    }   

}

However, recursion in such a case can potentially be bad. (If the search for the random variable happens to return "bad" values too many times, you might run into a stack overflow exception. The probability of this happening in this specific case is pretty low - but you should always consider such a case.)

Instead, you should go with an iterative approach, for example like this:

function changeParam($base_val)
{
    $plus = mt_rand(-25, 25);

    while (($base_val + $plus) > 255 || ($base_val + $plus) < 0) 
    {
        $plus = mt_rand(-25, 25);
    } 

    $new_val = $base_val + $plus;
    return $new_val;        
}

function changeParam($base_val)
{

    $plus = mt_rand(-25, 25);

    if (($base_val + $plus) > 255 || ($base_val + $plus) < 0) 
    {
        $new_val = changeParam($base_val);
    } 
    else 
    {   
        $new_val = $base_val + $plus;   
    }   

    // return here
    return $new_val;     

}