The documentation for 'offset' is misleading.

It says, "offset may be specified to begin searching an arbitrary number of characters into the string. Negative values will stop searching at an arbitrary point prior to the end of the string."

This is confusing if you think of strrpos as starting at the end of the string and working backwards.

A better way to think of offset is:

- If offset is positive, then strrpos only operates on the part of the string from offset to the end. This will usually have the same results as not specifying an offset, unless the only occurences of needle are before offset (in which case specifying the offset won't find the needle).

- If offset is negative, then strrpos only operates on that many characters at the end of the string. If the needle is farther away from the end of the string, it won't be found.

If, for example, you want to find the last space in a string before the 50th character, you'll need to do something like this:

strrpos($text, " ", -(strlen($text) - 50));

If instead you used strrpos($text, " ", 50), then you would find the last space between the 50th character and the end of the string, which may not have been what you were intending.

The description of offset is wrong. Here’s how it works, with supporting examples.

Offset effects both the starting point and stopping point of the search. The direction is always right to left. (The description wrongly says PHP searches left to right when offset is positive.)

Here’s how it works:
When offset is positive, PHP searches right to left from the end of haystack to offset. This ignores the left side of haystack.

When offset is negative, PHP searches right to left, starting offset bytes from the end, to the start of haystack. This ignores the right side of haystack.

Example 1:
$foo = ‘aaaaaaaaaa’;
var_dump(strrpos($foo, 'a', 5));
Result: int(10)

Example 2:
$foo = "aaaaaa67890";
var_dump(strrpos($foo, 'a', 5));
Result: int(5)

Conclusion: When offset is positive, PHP searches right to left from the end of haystack.

Example 3:
$foo = "aaaaa567890";
var_dump(strrpos($foo, 'a', 5));
Result: bool(false)

Conclusion: When offset is positive, PHP stops searching at offset.

Example 4:
$foo = ‘aaaaaaaaaa’;
var_dump(strrpos($foo, 'a', -5));
Result: int(6) 

Conclusion: When offset is negative, PHP searches right to left, starting offset bytes from the end. 

Example 5:
$foo = "a234567890";
var_dump(strrpos($foo, 'a', -5));
Result: int(0)

Conclusion: When offset is negative, PHP searches right to left, all the way to the start of haystack.

Ten years on, Brian's note is still a good overview of how offsets work, but a shorter and simpler summary is:

    strrpos($x, $y, 50);  // 1: this tells strrpos() when to STOP, counting from the START of $x
    strrpos($x, $y, -50); // 2: this tells strrpos() when to START, counting from the END of $x

Or to put it another way, a positive number lets you search the rightmost section of the string, while a negative number lets you search the leftmost section of the string.

Both these variations are useful, but picking the wrong one can cause some highly confusing results!

Here is a simple function to find the position of the next occurrence of needle in haystack, but searching backwards  (lastIndexOf type function):

//search backwards for needle in haystack, and return its position
function rstrpos ($haystack, $needle, $offset){
    $size = strlen ($haystack);
    $pos = strpos (strrev($haystack), $needle, $size - $offset);
    
    if ($pos === false)
        return false;
    
    return $size - $pos;
}

Note: supports full strings as needle