How to reverse words in a string?

Question

This task has already been asked/answered, but I recently had a job interview that imposed some additional challenges to demonstrate my ability to manipulate strings.

Problem: How to reverse words in a string? You can use strpos(), strlen() and substr(), but not other very useful functions such as explode(), strrev(), etc.

Example:

$string = "I am a boy"

Answer:

I ma a yob

Below is my working coding attempt that took me 2 days [sigh], but there must be a more elegant and concise solution.

Intention:

1. get number of words
2. based on word count, grab each word and store into array
3. loop through array and output each word in reverse order

Code:

$str = "I am a boy";

echo reverse_word($str) . "\n";

function reverse_word($input) {
    //first find how many words in the string based on whitespace
    $num_ws = 0;
    $p = 0;
    while(strpos($input, " ", $p) !== false) {
        $num_ws ++;
        $p = strpos($input, ' ', $p) + 1;
    }
    
    echo "num ws is $num_ws\n";
    
    //now start grabbing word and store into array
    $p = 0;
    for($i=0; $i<$num_ws + 1; $i++) {
        $ws_index = strpos($input, " ", $p);
        //if no more ws, grab the rest
        if($ws_index === false) {
            $word = substr($input, $p);
        }
        else {
            $length = $ws_index - $p;
            $word = substr($input, $p, $length);
        }
        $result[] = $word;
        $p = $ws_index + 1; //move onto first char of next word
    }
    
    print_r($result);
    //append reversed words
    $str = '';
    for($i=0; $i<count($result); $i++) {
        $str .= reverse($result[$i]) . " ";
    }
    return $str;
}

function reverse($str) {
    $a = 0;
    $b = strlen($str)-1;
    while($a < $b) {
        swap($str, $a, $b);
        $a ++;
        $b --;
    }
    return $str;
}

function swap(&$str, $i1, $i2) {
    $tmp = $str[$i1];
    $str[$i1] = $str[$i2];
    $str[$i2] = $tmp;
}

Answer 1

$string = "I am a boy";

$reversed = "";
$tmp = "";
for($i = 0; $i < strlen($string); $i++) {
    if($string[$i] == " ") {
        $reversed .= $tmp . " ";
        $tmp = "";
        continue;
    }
    $tmp = $string[$i] . $tmp;    
}
$reversed .= $tmp;

print $reversed . PHP_EOL;
>> I ma a yob