PHP: Fix read and write of float and double on big endian platforms (p…

…rotocolbuffers#13444)

Change the pack/unpack format codes 'f' -> 'g' and 'd' -> 'e' which ensure little endian format is used on the wire regardless of machine endianness. This allows the php composer and conformance tests to pass on big endian platforms and should not change any behavior on little endian platforms.

According to the PHP documentation, 'g' and 'e' were added in PHP versions 7.0.15 and 7.1.1.

Closes protocolbuffers#13444

COPYBARA_INTEGRATE_REVIEW=protocolbuffers#13444 from linux-on-ibm-z:php-endian-fix 7840e09
PiperOrigin-RevId: 553954880

php/src/Google/Protobuf/Internal/GPBWire.php

@@ -241,7 +241,7 @@ public static function readFloat(&$input, &$value)
         if (!$input->readRaw(4, $data)) {
             return false;
         }
-        $value = unpack('f', $data)[1];
+        $value = unpack('g', $data)[1];
         return true;
     }
 
@@ -251,7 +251,7 @@ public static function readDouble(&$input, &$value)
         if (!$input->readRaw(8, $data)) {
             return false;
         }
-        $value = unpack('d', $data)[1];
+        $value = unpack('e', $data)[1];
         return true;
     }
 
@@ -360,13 +360,13 @@ public static function writeBool(&$output, $value)
 
     public static function writeFloat(&$output, $value)
     {
-        $data = pack("f", $value);
+        $data = pack("g", $value);
         return $output->writeRaw($data, 4);
     }
 
     public static function writeDouble(&$output, $value)
     {
-        $data = pack("d", $value);
+        $data = pack("e", $value);
         return $output->writeRaw($data, 8);
     }