This script:

<?php
 $needle = "foo";
 echo "1: ";
 echo strstr("Moo _bar_", "_$needle_");
 echo "<br>2: ";
 echo strstr("Moo _bar_", "_{$needle}_");
 echo "<br>3: ";
 echo strstr("Moo _foo_", "_{$needle}_");
 echo "<br>4: ";
 echo strstr("Moo !bar!", "!$needle!");
?>

Gives this output:

1: _bar_
2: 
3: _foo_
4:

Obviously, the line 1 is the problem. If both "haystack" and "needle" have the strings surrounded by underscores - the strings are matched even if they are actually different. This happens only if the "needle" has a variable between the underscores, and the variable is not surrounded by { and }.

Prehaps I am missing something and the combination of "_$" is supposed to mean something, but i did not see it in the documentation.