bin2hex

This function is for converting binary data into a hexadecimal string representation. This function is not for converting strings representing binary digits into hexadecimal. If you want that functionality, you can simply do this:
<?php
$binary = "11111001";
$hex = dechex(bindec($binary));
echo $hex;
?>

This would output "f9". Just remember that there is a very big difference between binary data and a string representation of binary.

A good option for creating strings with binary data for saving (for example saving an sql statement to a file) into text files or php code is to do the following:
<?php
$field=bin2hex($field);
$field=chunk_split($field,2,"\\x");
$field= "\\x" . substr($field,0,-2);
?>

this will convert your field (binary or not) into hex and then convert the hex into a string which may be placed in a php file:
FFFFFFFF -> \xFF\xFF\xFF\xFF

I was just browsing the above and with a little modification,
came up with the following which I believe to be more flexible:
<?php
function bin2hex($data) {
 $corrected = ereg_replace("[^0-9a-fA-F]","",$data);
 return pack("H".strlen($corrected),$corrected);
}
?>

This will make sure that whatever you pass, even if it is padded
at the extremeties or between pairs, should return the desired data.

Convenient way of generating API keys
<?php
$apikey = bin2hex(random_bytes(32)); // generates 64 characters long string /^[0-9a-f]{64}$/
?>

In an attempt to dodge spam bots I've seen people (including myself) hex encode their email addresses in "mailto" tags. This is the small chunk of code I wrote to automate the process:
<?php
function hex_encode ($email_address) {
 $encoded = bin2hex("$email_address");
 $encoded = chunk_split($encoded, 2, '%');
 $encoded = '%' . substr($encoded, 0, strlen($encoded) - 1);
 return $encoded; 
}
?>

so for example:
<a href="mailto:&lt;?=hex_encode("pedram@redhive.com")?>">email me</a>
would produce the following address:
%70%65%64%72%61%6d%40%72%65%64%68%69%76%65%2e%63%6f%6d
-pedram

Here's a function to check if a string contains any 7-bit GSM characters.
It might come useful for people working on SMS platforms.
<?php
 
function check_gsm($str)
{
 $arr = array(
"0x00", "0x01", "0x02", "0x03", "0x04", "0x05","0x06","0x07","0x08","0x09",
"0x0A","0x0B","0x0C","0x0D","0x0E","0x0F","0x10","0x11","0x12","0x13",
"0x14","0x15","0x16","0x17","0x18","0x19","0x1A","0x1B","0x1B0A",
"0x1B14","0x1B28","0x1B29","0x1B2F","0x1B3C","0x1B3D","0x1B3E",
"0x1B40","0x1B65","0x1C","0x1D","0x1E","0x1F","0x20","0x21","0x22",
"0x23","0x24","0x25","0x26","0x27","0x28","0x29","0x2A","0x2B","0x2C",
"0x2D","0x2E","0x2F","0x30","0x31","0x32","0x33","0x34","0x35","0x36",
"0x37","0x38","0x39","0x3A","0x3B","0x3C","0x3D","0x3E","0x3F","0x40",
"0x41","0x42","0x43","0x44","0x45","0x46","0x47","0x48","0x49","0x4A",
"0x4B","0x4C","0x4D","0x4E","0x4F","0x50","0x51","0x52","0x53","0x54",
"0x55","0x56","0x57","0x58","0x59","0x5A","0x5B","0x5C","0x5D","0x5E",
"0x5F","0x60","0x61","0x62","0x63","0x64","0x65","0x66","0x67","0x68",
"0x69","0x6A","0x6B","0x6C","0x6D","0x6E","0x6F","0x70","0x71","0x72",
"0x73","0x74","0x75","0x76","0x77","0x78","0x79","0x7A","0x7B","0x7C",
"0x7D","0x7E","0x7F");
 
 $strl = strlen($str);
 for ($i = 0;$i < $strl; $i++)
 {
 $char = '0x' . bin2hex(substr($str,$i,1));
 $pos = in_array($char,$arr);
 if ($pos == 1)
 {
 $j++;
 }
 }
 
 if ($j < $strl)
 {
 return false;
 }
 else
 {
 return true;
 }
}
?>