Introduction
You've probably heard of "Magic Methods" if you've done anything at all with OOP in PHP. Most likely the __construct() method is invoked when initializing an instance of a class you've defined.
While __construct() is unofficially a magic method that is required to be defined when writing classes, __toString() is optional in every sense of the word and is often overlooked in favor of writing methods designed to be invoked for the sole reason of returning a human-readable string.
There's nothing wrong with doing that, but if you can get your objects to return a human-readable string without typing anything more than the $object reference instead of $object->method(), why not get it to do that?
The concepts in this tutorial apply to PHP version 5.2 or newer. Because of limitations in the context that PHP invoked the __toString magic method before PHP 5.2, the examples contained in this tutorial may not work in those older versions.
Examples of Usage
Let's compare an example using both common practice, and using __toString.
First, an example using common practice to load a CSV file and print it as an HTML <table>.
<?php
$data_table = new csv_html('file.csv');
echo '<h2>Data Table</h2>' . $data_table->html_table();
?>
Now let's look at that same example if __toString had been defined in place of the html_table() method.
<?php
$data_table = new csv_html('file.csv');
echo "<h2>Data Table</h2>$data_table";
?>
A more useful example might be anonymous instantiation, where no reference to the object is kept. This is useful where an object is a simple formatter and will likely never be manipulated after instantiation.
<?php
printf('<h2>Data Table</h2>%s', new csv_html('file.csv'));
?>
So How do I Implement __toString()?
Precisely the same way you define your __construct() method, and any of the other magic methods, with the exception of __autoload().
As a public method within your class definition.
Here's an example, albeit a crude one, of how the class in the CSV example might be written.
<?php
class csv_html
{
public $filename, $csv;
public function __construct($filename)
{
$this->filename = $filename;
$this->csv = file_get_contents($filename);
return $this;
}
public function __toString()
{
return sprintf(
'<table><tr><td>%s</td></tr></table>',
str_replace(array("\n", ','), array('</tr><tr>', '</td><td>'), $this->csv)
);
}
}
?>
Now you might ask, "Why not just have the constructor return the HTML?".
Because it would make what will become your parent class constructor useless if you ever tried to extend the class, that's why.
Conclusion
Give __toString() a try, you might like it.
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