可以在while循环中声明变量吗？

Question

I am trying to understand the basic PHP code in this video, which demonstrates how you can list all the positions of a specific string within a larger string (in this case, all the positions of $find within $string will be listed):

<?php

$find = 'is';
$find_leng = strlen($find);
$string = 'This is a string, and it is an example.';

while ($string_position = strpos($string, $find, $offset)) {
    echo '<strong>'.$find.'</strong> found at '.$string_position.'<br>';
    $offset = $string_position + $find_length;
}

?>

What confuses me is that within the () of the while-loop, it seems that a new variable $string_position is being declared. But doesn't the while-loop take an input of 0 or 1, a Boolean? The $string_position variable is not a true/false variable, it is meant to store the position of the string it is passed with the strpos() function.

The basic while-loop I'm used to uses a comparison operator like this one from from w3schools:

<?php 
$x = 1; 

while($x <= 5) {
    echo "The number is: $x <br>";
    $x++;
} 
?>

Can a variable be declared in a while-loop? And if that is the case, how does it work in this example? This is my first question on Stack Overflow so hopefully I'm posting this in the right place and it's not too newbie of a question.

Answer 1

Yes, while loops take boolean-like as arguments. By doing

while ($string_position = strpos($string, $find, $offset)) {

it first executes what is inside the parenthesis, then evaluates the result of that to determine if the loop should quit or not.

strpos will return False if the needle was not found, so $string_position will be false and as such the evaluation result will also be False and the loop will quit. If the needle is found, then loop continues after the attribution.

Also it should be noted that strpos might return a non-boolean that evaluates to False, so be careful with that construction.

Reference on strpos