var_dump(strpos("haystack", "hay")); // int(0)
var_dump(strpos("haystack", "stack")); // int(3)
var_dump(strpos("haystack", "stackoverflow"); // bool(false)

$pos = strpos("abcd", "a"); // $pos = 0;
$pos2 = strpos("abcd", "e"); // $pos2 = FALSE;

// Bad example of checking if a needle is found.
if($pos) { // 0 does not match with TRUE.
    echo "1. I found your string\n";
}
else {
    echo "1. I did not found your string\n";
}

// Working example of checking if needle is found.
if($pos !== FALSE) {
    echo "2. I found your string\n";
}
else {
    echo "2. I did not found your string\n";
}

// Checking if a needle is not found
if($pos2 === FALSE) {
    echo "3. I did not found your string\n";
}
else {
    echo "3. I found your string\n";
}

1. I did not found your string 
2. I found your string 
3. I did not found your string

// With offset we can search ignoring anything before the offset
$needle = "Hello";
$haystack = "Hello world! Hello World";

$pos = strpos($haystack, $needle, 1); // $pos = 13, not 0

$haystack = "a baby, a cat, a donkey, a fish";
$needle = "a ";
$offsets = [];
// start searching from the beginning of the string
for($offset = 0;
        // If our offset is beyond the range of the
        // string, don't search anymore.
        // If this condition is not set, a warning will
        // be triggered if $haystack ends with $needle
        // and $needle is only one byte long.
        $offset < strlen($haystack); ){
    $pos = strpos($haystack, $needle, $offset);
    // we don't have anymore substrings
    if($pos === false) break;
    $offsets[] = $pos;
    // You may want to add strlen($needle) instead,
    // depending on whether you want to count "aaa"
    // as 1 or 2 "aa"s.
    $offset = $pos + 1;
}
echo json_encode($offsets); // [0,8,15,25]