Convert mobile number to international format dynamically with php

I often need to covert mobile numbers into a UK international format of 44 followed by the number such as 442548951486 the numbers I work with can be in a mixed format some without a starting 0 others with 44 and same with spaces, so I wrote function that will convert a number in any mixture into a standard format quick and easily and as all the code is inside a function is reusable which means converting numbers in bulk is now very easy to do.

The function simple needs a number passing to it, a check is done to make sure the number only contains numbers then a check is done to see if the number starts with a 0 if not if will add a 0 as long as the start is not 4. Then a check is done to replace the 0 with a 44 then finally any spaces are removed and the formatted number is then returned.

Here's the function:

<?php
function format_number($number)
{
    //make sure the number is actually a number
    if(is_numeric($number)){

        //if number doesn't start with a 0 or a 4 add a 0 to the start.
        if($number[0] != 0 && $number[0] != 4){
            $number = "0".$number;
        }

        //if number starts with a 0 replace with 4
        if($number[0] == 0){
            $number[0] = str_replace("0","4",$number[0]);
            $number = "4".$number;
        }

        //remove any spaces in the number
        $number = str_replace(" ","",$number);

        //return the number
        return $number;

    //number is not a number
    } else {

        //return nothing
        return false;
    }
}
?>

<p>Usage Example:</p>

echo format_number('07295514973');