Notice: A non well formed numeric value encountered in C:\xampp\htdocs\Space\Find\index.php on line 21

Answer 1

$offset is supposed to be numerical, but you are assigning a string to it in the while itself:

echo $offset = $stringpos + $search_length.'<br/>';

After this line, $offset is no longer a number and the second time the while is executed, you'd get that notice

Answer 2

while($strpos=strpos($text,$search,(int)($offset))){
    echo $offset=($strpos+$search_length)."<br>";
}

Need a type casting in while loop... (int)($offset) That's it.