Navigating within Files - Page 5

In Part 1, we looked at opening a file, and reading it either line by line, or character by character. In this section, we look at moving around to any position in the file.

When you open a file, PHP keeps track of something called the file pointer. Normally (unless opening in append mode), this points to the beginning of the file 9 which is 0). When you read a file with the fgetc() function, the file pointer is incremented by one. We can see this in action by using the ftell() function, which returns the position in the file.

int ftell (int fp)
In Part 1, we've already seen the feof() function, which returns true if the file pointer is at the end of the file. The following script take a single line text file, and displays the movement of the file pointer. Let's assume that the file potofgold.txt contains a single line containing the characters - 123456789 and a newline at the end:

<?
$filename = "potofgold.txt";
if (!($fp = fopen ($filename, "r"))) {
print "Error - could not open the file $filename<br>";
}
else {
 while (!feof ($fp)) {
  $file_pointer = ftell($fp);
  print "File pointer: $file_pointer<br>";
  $file_char = fgetc($fp);
  // do something with $file_char...
 }
 fclose ($fp);
}
?>

The results would be displayed as follows:

File pointer: 0
File pointer: 1
File pointer: 2
File pointer: 3
File pointer: 4
File pointer: 5
File pointer: 6
File pointer: 7
File pointer: 8
File pointer: 9
File pointer: 10

The file pointer starts at 0, then moves up through characters 1-9, finally moving to 10 after the newline character, which is also counted as a valid character.

The real power comes from being able to manipulate the file pointer directly. The rewind() function returns the file pointer to the beginning of the file, while the fseek() function allows you to move the pointer to any location in the file. The rewind() function just take the file handle as an argument:
int rewind (int fp)

The fseek() function takes a file handle, an offset (how much to move the file pointer), and optionally, an argument indicating where to take the offset from:
int fseek (int fp, int offset [, option])
Option can be one of SEEK_SET (the default, which sets the offset from the start of the file), SEEK_CUR (sets the offset from the current position of the file pointer), and SEEK_END (sets the offset from the end of the file). The offset can be a positive or a negative number (for counting backwards, especially from the end of the file). The following script demonstrates some of these features

<?
$filename = "potofgold.txt";
if (!($fp = fopen ($filename, "r"))) {
 print "Error - could not open the file $filename<br>";
}
else {
 while (!feof ($fp)) {
  $file_pointer = ftell($fp);
  print "File pointer: $file_pointer<br>";
  $file_char = fgetc($fp);
  // do something with $file_char...
 }
 rewind($fp);
 $file_pointer = ftell($fp);
 print "File pointer after rewind: $file_pointer<br>";
 fseek($fp,3);
 $file_pointer = ftell($fp);
 print "File pointer after fseek(\$fp,3): $file_pointer<br>";
 fseek($fp,1,SEEK_CUR);
 $file_pointer = ftell($fp);
 print "File pointer after fseek(\$fp,1,SEEK_CUR): $file_pointer<br>";
 fseek($fp,-1,SEEK_END);
 $file_pointer = ftell($fp);
 print "File pointer after fseek(\$fp,-1,SEEK_END): $file_pointer<br>";
 fclose ($fp);
}
?>

This displays:

File pointer: 0
File pointer: 1
File pointer: 2
File pointer: 3
File pointer: 4
File pointer: 5
File pointer: 6
File pointer: 7
File pointer: 8
File pointer: 9
File pointer: 10
File pointer after rewind: 0
File pointer after fseek($fp,3): 3
File pointer after fseek($fp,1,SEEK_CUR): 4
File pointer after fseek($fp,-1,SEEK_END): 9

A warning though - it's easy to make a the common mistake of getting stuck in an endless loop. For example, by putting the rewind inside the while loop, feof() will never return a true value, and the while loop will never end. Don't run this script below!

<?
 // WARNING - don't run this - causes an endless loop!
$filename = "potofgold.txt";
if (!($fp = fopen ($filename, "r"))) {
 print "Error - could not open the file $filename";
}
else {
 while (!feof ($fp)) {
  $file_pointer = ftell($fp);
  print "File pointer: $file_pointer
";
  $file_char = fgetc($fp);
  // do something with $file_char...

  // WARNING - don't run this - causes an endless loop!
  rewind($fp);
  
 }
 fclose ($fp);
}
?>

Handling files with PHP4 - Part 2 - Page 4
Handling files with PHP4 - Part1
Working with directories - Page 6