No, I mean besides taking a string full of code as one of its arguments.

Consider this contrived example:

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<?php
$input = array(1, 2, 3);
 
$monolithic_dimensions = array_map(
    create_function('$elem', 'return $elem * $elem;'),
    $input
);
 
/* Prints Array([0] => 1, [1] => 4, [2] => 9) */
print_r($monolithic_dimensions);
?>

On the face of it, this looks like a decent approximation of functional programming idiom. However, every time PHP executes create_function, it adds a new function to the global function table.

What’s really happening is a little easier to see if you look at create_function’s return value:

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<?php
 
$square = create_function('$elem', 'return $elem * $elem;');
 
print gettype($square) . "\n"; /* Prints string */
 
/* Prints
0  []
108	[l]
97	[a]
109	[m]
98	[b]
100	[d]
97	[a]
95	[_]
49	[1]
*/
for ($i = 0; $i < strlen($square); $i++) {
    print ord($square[$i]) . "\t[" . $square[$i] . "]\n";
}
 
?>

Yup. PHP is creating a function at runtime, with an “impossible” name (the zero byte isn’t allowed in function statements) to avoid collisions with user-defined functions.

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<?php
while ($read < $n && (false !== ($buf = fread($this->sock, $n - $read)))) {
    /* ... */
}
?>

contains a subtle bug. Go ahead, read the relevant function documentation and see if you can spot it.

PHP’s fread function directly maps the C library’s fread function. As TBlue explains, the C fread function returns 0 when reading at the end of a stream. PHP does the simplest possible thing with this and returns an empty string; to determine that the stream is at end of file, programs must check with feof as well.

Most modern languages realize that explicitly checking for errors is a fairly error-prone convention. Java’s InputStream.read(byte[]) methods return -1 on EOF (and only on EOF) and raise exceptions on errors. Python’s File-like objects convention supports mechanisms other than raw byte reads (primarily, iteration) that make stopping at end of file implicit, on top of only returning an empty string at end of input (as with Java, Python raises exceptions when a stream read fails). Not PHP, though! In PHP, the loop above has to look like

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<?php
while ($read < $n && !feof($this->sock) && (false !== ($buf = fread($this->sock, $n - 
$read)))) {
    /* ... */
}
?>

Encountered in the wild: php-amqplib bug #12.