PHP - How can I convert a large hexadecimal value to a binary value?

You need to use hex2bin();  Pass in your hex number and it will convert it to binary

I tried that prior to the base_convert() function and it also does not work.  In fact, the PHP documentation says the following about the hex2bin() function:

CAUTION This function does NOT convert a hexadecimal number to a binary number. This can be done using the base_convert() function.

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You got the order wrong.  This works.

<?php
   $hexString = "100000000000000000";
   echo base_convert( $hexString, 2, 16 );
?>

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The order I placed the parameters is correct (from the PHP manual:

string base_convert ( string $number , int $frombase , int $tobase )

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I am converting a hex value of "100000000000000000" to a binary value

Ok, I rewrote my test program to include a lot of known values working up to your desired input.  It appears that you are exceeding the resolution of the function.

<?php
   $hexString = "1";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "2";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "4";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "8";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "A";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "F";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "10";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "100";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "1000";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "10000";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "1000000";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "100000000";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "1000000000000000";
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
   $hexString = "100000000000000000"; // your number
   echo "$hexString H = ".base_convert( $hexString, 16, 2 )." B<br>";
?>

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This page has comments about exceeding the resolution.  http://php.net/manual/en/function.base-convert.php

Thanks NerdsOfTech

Thank you. I cleaned the code up a bit further in case you want a UDF for this task (TESTED):

function hexbin64plus($str, $reverse = 0){
 $str = preg_replace("/[^0-9A-Fa-f]/", "", $str); // sanitize all non-hex chars
 if(preg_match("/[a-f]/i", $str) == 1){ // hex detected
   $str = strtolower($str); // coalesce A-F to a-f
 }
 if($reverse == 0){ // hex to bin (default)
  $chunk = 1;
  $frombase = 16;
  $tobase = 2;
  $format = '%04d';
 }else{ // bin to hex (reverse flag is on)
  $chunk = 4;
  $frombase = 2;
  $tobase = 16;
  $format = '%s';
 }
 $chars = str_split($str, $chunk);
 $str = '';
 foreach($chars as $char){
  $char = sprintf($format, base_convert($char, $frombase, $tobase));
  $str .= $char;	
 }
 return $str;
}

$hex = $bin = '';
$hex = '100000000000000000';
echo $hex . " H <BR>";
$bin = hexbin64plus($hex);
echo ltrim($bin, '0') . " B" . "<BR>";
echo strlen(ltrim($bin, '0')) . " bits" . "<BR>";

// reverse the conversion
$hex = hexbin64plus($bin, 1);
echo strtoupper($hex) . " H" . "<BR>";
echo "<HR><BR>";

//try another conversion
$hex = '2A';
echo $hex . " H <BR>";
$bin = hexbin64plus($hex);
echo ltrim($bin, '0') . " B" . "<BR>";
echo strlen(ltrim($bin, '0')) . " bits" . "<BR>";

// reverse the second conversion
$hex = hexbin64plus($bin, 1);
echo strtoupper($hex) . " H" . "<BR>";

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