ITEC 325 2018spring ibarland

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php’s slippery types

introduce the Project

summary

• Call error_reporting(E_ALL);, in every file.
• Use === (and, !==) to compare, in php.
• The functions is_numeric, is_string, etc. are ones you actually want to use regularly (rather than type-casts, explicit or implicit).

There are some things that are not an error, but should be. For example: echo $iNeverDefinedThisVariable + 47; or echo $notAnArray[9+$notAnIndex] + 47; or echo strlen(heyThisHasNoQuotesSoIsNotReallyAString);.

This means that if you ever mis-type a variable name, you won't get an error, but instead PHP will happily proceed as if nothing is wrong¹.
PHP does not have your back.

Solution: Fortunately, people figured out that PHP's default behavior is unforgiveable, and added a switch to turn the above three behaviours into errors, as they should be: error_reporting(E_ALL);. Put this line at the top of all your PHP files, and save hours of headaches later.

Types in php

PHP is dynamically typed: that is, it figures out the type of values at run time — whether an expression evaluates to a boolean, int, float, string, arrays, or object. (Contrast that with Java, which is statically typed: it knows the type of each value at compile-time, since the programmer is required to state the type each variable will hold, and the type returned by a function, and the type that must be passed in to each function.) So a PHP variable can hold an int at one moment in time, and later hold a string; a function might return a string sometimes and return a boolean other times, etc.. Javascript, python, and racket are other dynamically-typed languages.

aside: Some people say that PHP is “untyped”; that is wrong. It is always aware of the type of any value. PHP has the types you might expect, including boolean, int, float, string, arrays, objects.² Sometimes people (erroneously) say PHP is “weakly typed”. They might be trying to say “dynamically typed”, or they might be trying to say “PHP is particularly fast-and-loose about type conversions.”. It is true that PHP does significantly more “type juggling” than most sane languages.

Conversion between strings and integers:

echo "hello" + "world";
echo 12 + 3;
echo "12" + "3";
echo "12" + 3;
echo "12hello" + "3world7" + "nevermind92";

echo strlen(37);
     

Rationale:

The solution: If you have a string, use is_numeric along with type casting.

Booleans: Many values, if used in a boolean context, are "false-ish":

echo false == false;
echo false == 0;
echo false == 0.0;
echo false == "0";
echo false == array();
echo $someUndefinedVar == false;   // ?!
    

echo false == 0;           // true, as just noted.
// == is not transitive (!):
echo 0 == "00";            // true.
echo false == "00";        // false?!

echo 0 == "-0";            // true.
echo false == "-0";        // false?!

echo 0 == 0.0;             // true.
echo false == "0.0";       // false?!
    

Caution: There are also the boolean operators with spelled-out names, AND and OR. Do not use them! They are just like &&, || except that they have unexpectedly low precedence (less than the assignment operators³ Use parentheses to force precedence if you are doing something unusual (or even, just use parentheses routinely, so that you never even need to think twice about precedence!).

Upshot: PHP has ===, which does comparison suppressing any type-casts. Using this avoids unexpected results; when you want to convert you can call conversion-functions explicitly.

• So, for this class (and in all php), use === (and, !==) to compare, in php.
• Sure, you'll see examples on the web and in books that use ==, but that's just asking for trouble.
• (If using objects, though: php's === tests for identical objects (like Java's ==), and php's == does a one-level-deep-equals (closer to Java's .equals?).)

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