In PHP what does it mean by a function being binary-safe?

It means the function will work correctly when you pass it arbitrary binary data (i.e. strings containing non-ASCII bytes and/or null bytes).

For example, a non-binary-safe function might be based on a C function which expects null-terminated strings, so if the string contains a null character, the function would ignore anything after it.

This is relevant because PHP does not cleanly separate string and binary data.

The other users already mentioned what binary safe means in general.

In PHP, the meaning is more specific, referring only to what Michael gives as an example.

All strings in PHP have a length associated, which are the number of bytes that compose it. When a function manipulates a string, it can either:

1. Rely on that length meta-data.
2. Rely on the string being null-terminated, i.e., that after the data that is actually part of the string, a byte with value 0 will appear.

It's also true that all string PHP variables manipulated by the engine are also null-terminated. The problem with functions that rely on 2., is that, if the string itself contains a byte with value 0, the function that's manipulating it will think the string has ended at that point and will ignore everything after that.

For instance, if PHP's strlen function worked like C standard library strlen, the result here would be wrong:

$str = "abc\x00abc";
echo strlen($str); //gives 7, not 3!

More examples:

<?php

    $string1 = "Hello";
    $string2 = "Hello\x00World";

    // This function is NOT ! binary safe
    echo strcoll($string1, $string2); // gives 0, strings are equal.

    // This function is binary safe
    echo strcmp($string1, $string2); // gives <0, $string1 is less than $string2.

?>

\x indicates hexadecimal notation. See: PHP strings

0x00 = NULL
0x04 = EOT (End of transmission)

ASCII table to see ASCII char list