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4.2. Printing Strings

There are four ways to send output to the browser. The echo construct lets you print many values at once, while print( ) prints only one value. The printf( ) function builds a formatted string by inserting values into a template. The print_r( ) function is useful for debugging—it prints the contents of arrays, objects, and other things, in a more-or-less human-readable form.

4.2.3. printf( )

The printf( ) function outputs a string built by substituting values into a template (the format string). It is derived from the function of the same name in the standard C library. The first argument to printf( ) is the format string. The remaining arguments are the values to be substituted in. A % character in the format string indicates a substitution.

4.2.3.2. Type specifiers

The type specifier tells printf( ) what type of data is being substituted. This determines the interpretation of the previously listed modifiers. There are eight types, as listed in Table 4-2.

Table 4-2. printf( ) type specifiers

Specifier

Meaning

B

The argument is an integer and is displayed as a binary number.

C

The argument is an integer and is displayed as the character with that value.

d or I

The argument is an integer and is displayed as a decimal number.

e, E, or f

The argument is a double and is displayed as a floating-point number.

g or G

The argument is a double with precision and is displayed as a floating-point number.

O

The argument is an integer and is displayed as an octal (base-8) number.

S

The argument is a string and is displayed as such.

U

The argument is an unsigned integer and is displayed as a decimal number.

x

The argument is an integer and is displayed as a hexadecimal (base-16) number; lowercase letters are used.

X

The argument is an integer and is displayed as a hexadecimal (base-16) number; uppercase letters are used.

The printf( ) function looks outrageously complex to people who aren't C programmers. Once you get used to it, though, you'll find it a powerful formatting tool. Here are some examples:

The sprintf( ) function takes the same arguments as printf( ) but returns the built-up string instead of printing it. This lets you save the string in a variable for later use:

$date = sprintf("%02d/%02d/%04d", $month, $day, $year);
// now we can interpolate $date wherever we need a date

4.2.4. print_r( ) and var_dump( )

The print_r( ) construct intelligently displays what is passed to it, rather than casting everything to a string, as echo and print( ) do. Strings and numbers are simply printed. Arrays appear as parenthesized lists of keys and values, prefaced by Array:

$a = array('name' => 'Fred', 'age' => 35, 'wife' => 'Wilma');
print_r($a);
Array
(
    [name] => Fred
    [age] => 35
    [wife] => Wilma
)

Using print_r( ) on an array moves the internal iterator to the position of the last element in the array. See Chapter 5 for more on iterators and arrays.

When you print_r( ) an object, you see the word Object, followed by the initialized properties of the object displayed as an array:

class P {
  var $name = 'nat';
  // ...
}

$p = new P;
print_r($p);
Object
(
    [name] => nat
)

Boolean values and NULL are not meaningfully displayed by print_r( ):

print_r(true);       print "\n";
1
print_r(false);      print "\n";

print_r(null);       print "\n";

For this reason, var_dump( ) is preferable to print_r( ) for debugging. The var_dump( ) function displays any PHP value in a human-readable format:

var_dump(true);
bool(true)
var_dump(false);
bool(false);
var_dump(null);
bool(null);
var_dump(array('name' => Fred, 'age' => 35));
array(2) {
  ["name"]=>
  string(4) "Fred"
  ["age"]=>
  int(35)
}
class P {
  var $name = 'Nat';
  // ...
}
$p = new P;
var_dump($p);
object(p)(1) {
  ["name"]=>
  string(3) "Nat"
}

Beware of using print_r( ) or var_dump( ) on a recursive structure such as $GLOBALS (which has an entry for GLOBALS that points back to itself ). The print_r( ) function loops infinitely, while var_dump( ) cuts off after visiting the same element three times.



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