PHP, Arrays & Passing by Reference
Do you know that objects in PHP5 are passed by reference, while arrays and other scalar variables are passed by value? Yes, you know it, but it’s not exactly true. Let’s see some example and let’s try to answer few questions.
// depending on the machine but both lines return
// expectedly equal values: 331208
These two lines of code, expectedly return the same value (in my case 331208), which shows us that because nothing happened in between them the memory usage isn’t growing. Let’s now put some code in between them.
echo memory_get_usage(); // 331616
$a = 10;
echo memory_get_usage(); // 331696
Let’s say we have an array of data represented by some text followed by a number. Just like the movies from a movie series like “Mission Impossible” or “Pirates of the Carribean”. We know that they are often followed by the consecutive number of the episode.
Mission: Impossible 1
Mission: Impossible 2
Mission: Impossible 3
Since we have no more than three or four episodes we can easily sort the array if it’s not sorted initially.
$a = array('Mission: Impossible 2', 'Mission: Impossible 3', 'Mission: Impossible 1');
// Mission: Impossible 1
// Mission: Impossible 2
// Mission: Impossible 3
However in some cases we can have more than 10 episodes. Then we can meet a problem while sorting the array above.
$a = array('Episode 1', 'Episode 2', 'Episode 11', 'Episode 112');
// Episode 1
// Episode 11
// Episode 112
// Episode 2
Now because this is by default an alphabetical sort order we get an array that isn’t sorted to our human undestanding.
Alphabetical vs. Natural sort order
The question is how to overcome this problem?
I bet you didn’t know that PHP strings don’t need quotes! Indeed PHP developers work with strings with either single or double quotes, but actually in some cases you don’t need them.
PHP by Book
Here’s how PHP developer declare a string, which is something very common in any programming language.
$my_var = 'hello world';
$my_var = "hello world";
What if you do the following:
That appears to be correct … Well, it’s not absolutely correct. You’ll be “noticed”.
// Notice: Use of undefined constant hello
However if you disable error reporting, the code will be completely fine.
// no problem now
What follows from the thing above is that you can use strings without quotes:
// hello world (concatenated)
echo hello . ' world';
echo hello . world;
However you can’t have spaces and most of the “special” symbols.
// syntax error
echo hello world;
// syntax error
Although you can do this in PHP, that is completely wrong. The code becomes more difficult to read and understand. In the second place you can miss a $ sign in front of a variable declaration and thus the PHP interpreter will assume this is a string. So disable error reporting isn’t so great sometimes.
Sorted data can dramatically change the speed of our program, therefore sorting algorithms are something quite special in computer science. For instance searching in a sorted list is faster than searching in an unordered list.
There are two main approaches in sorting – by comparing the elements and without comparing them. A typical algorithm from the first group is insertion sort. This algorithm is very simple and very intuitive to implement, but unfortunately it is not so effective compared to other sorting algorithms as quicksort and merge sort. Indeed insertion sort is useful for small sets of data with no more than about 20 items.
Insertion sort it is very intuitive method of sorting items and we often use it when we play card games. In this case the player often gets an unordered set of playing cards and intuitively starts to sort it. First by taking a card, making some comparisons and then putting the card on the right position.
So let’s say we have an array of data. In the first step the array is unordered, but we can say that it consists of two sub-sets: sorted and unordered, where on the first step the only item in the sorted sub-set is its first item. If the length of the array is n the algorithm is considered completed in n-1 steps. On each step our sorted subset is growing with one item. The thing is that we take the first item from the unordered sub-set and with some comparisons we put it into its place in the sorted sub-set, like on the diagram bellow.
Main principle of insertion sort.
The Typical PHP Approach
Typically a PHP programmer will write his SQL query as a string and will execute it via mysql_query.
$sql = "SELECT * FROM my_table";
$resource = mysql_query($sql);
So eventually when you want to dump this “complex” query, or whatever query there is, you can simply “echo” it and see what’s its syntax.
// this query is WRONG because of the where clause
$sql = "SELECT * FROM my_table WHERE id = ";
// dump and debug the wrong query
// this line won't be executed
$resource = mysql_query($sql);
So far so good, but things appear to be a bit different when you start to work with Zend Framework. Higher levels of abstraction come with slightly more difficult ways to dump (debug) your SQL queries.
OK you’ve two options. Using Zend_Db_Select or … not.