PHP Strings Storing Binary Data in Strings - Web Development and Design | Tutorial for Java, PHP, HTML, Javascript PHP Strings Storing Binary Data in Strings - Web Development and Design | Tutorial for Java, PHP, HTML, Javascript

Breaking

Post Top Ad

Post Top Ad

Wednesday, May 8, 2019

PHP Strings Storing Binary Data in Strings

PHP Strings





Storing Binary Data in Strings


Problem

You want to parse a string that contains values encoded as a binary structure or encode values into a string. For example, you want to store numbers in their binary representation instead of as sequences of ASCII characters.


Solution

Use pack() to store binary data in a string:

              $packed = pack('S4',1974,106,28225,32725);

Use unpack() to extract binary data from a string:

              $nums = unpack('S4',$packed);


Discussion

The first argument to pack() is a format string that describes how to encode the data that’s passed in the rest of the arguments. The format string S4 tells pack() to produce four unsigned short 16-bit numbers in machine byte order from its input data. 

Given 1974, 106, 28225, and 32725 as input on a little-endian machine, this returns eight bytes: 182, 7, 106, 0, 65, 110, 213, and 127. Each two-byte pair corresponds to one of the input numbers: 7 * 256 + 182 is 1974; 0 * 256 + 106 is 106; 110 * 256 + 65 = 28225; 127 * 256 + 213 = 32725.

The first argument to unpack() is also a format string, and the second argument is the
data to decode. Passing a format string of S4, the eight-byte sequence that pack()
produced returns a four-element array of the original numbers. print_r($nums) prints:

               Array
               (
                        [1] => 1974
                        [2] => 106
                        [3] => 28225
                        [4] => 32725
               )

In unpack(), format characters and their count can be followed by a string to be used as an array key. For example:

               $nums = unpack('S4num',$packed);
               print_r($nums);

This prints:

               Array
               (
                       [num1] => 1974
                       [num2] => 106
                       [num3] => 28225
                       [num4] => 32725
               )

Multiple format characters must be separated with / in unpack():

               $nums = unpack('S1a/S1b/S1c/S1d',$packed);
               print_r($nums);

This prints:

               Array
               (
                       [a] => 1974
                       [b] => 106
                       [c] => 28225
                       [d] => 32725
               )

Format characters for pack( ) and unpack( )

Format character  Data type
a                              NUL-padded string
A                             Space-padded string
h                              Hex string, low nibble first
H                             Hex string, high nibble first
c                              signed char
C                             unsigned char
s                               signed short (16 bit, machine byte order)
S                              unsigned short (16 bit, machine byte order)
n                              unsigned short (16 bit, big endian byte order)
v                              unsigned short (16 bit, little endian byte order)
i                               signed int (machine-dependent size and byte order)
I                               unsigned int (machine-dependent size and byte order)
l                               signed long (32 bit, machine byte order)
L                              unsigned long (32 bit, machine byte order)
N                             unsigned long (32 bit, big endian byte order)
V                             unsigned long (32 bit, little endian byte order)
f                               float (machine-dependent size and representation)
d                              double (machine-dependent size and representation)
x                              NUL byte
X                             Back up one byte
@                            NUL-fill to absolute position

For a, A, h, and H, a number after the format character indicates how long the string is. For example, A25 means a 25-character space-padded string. For other format characters, a following number means how many of that type appear consecutively in a string. Use * to take the rest of the available data.

You can convert between data types with unpack(). This example fills the array $as cii with the ASCII values of each character in $s:

                $s = 'platypus';
                $ascii = unpack('c*',$s);
                print_r($ascii);

This prints:

                 Array
                 (
                        [1] => 112
                        [2] => 108
                        [3] => 97
                        [4] => 116
                        [5] => 121
                        [6] => 112
                        [7] => 117
                        [8] => 115
                 )


No comments:

Post a Comment

Post Top Ad