eval() not working?

Question

I'm trying to get the focal length from an image's EXIF data via PHP.

This is the code I've got so far:

$exif = exif_read_data("$photo");
$length10 = $exif['FocalLength'];
$length = eval($length10);

$length10 in this case returns something like "1050/10" for 105mm. I don't know why. All I want to do is have PHP do the math to return 105. When I run this, though, I get the following error message:

[04-Nov-2012 20:06:39] PHP Parse error:  syntax error, unexpected $end in index.php(52) : eval()'d code on line 1

Why?

Answer 1

Because 1050/10 is not valid PHP. It has no terminating ; to end the statement, and results in a syntax error.

php > eval("1050/10");
PHP Parse error:  syntax error, unexpected end of file in php shell code(1) : eval()'d code on line 1

Rather than eval() it (which technically is dangerous since you're effectively processing user input even if it comes from EXIF), it is recommended to split on the / or capture the operands with a regular expression and then perform the operation yourself.

// Test if the value matches the division pattern
if (preg_match('~^(\d+)/(\d+)$~', $length10, $operands)) {
  // Following a successful match, $operands is an array 
  // containing the full matched string and the two numbers captured
  // in indices [1],[2]

  // Watch for div by zero!
  if ($matches[2] !== 0) {
    echo $operands[1] / $operands[2];
  }
}
else {
  echo $length10;
}