putenv/getenv, $_ENV, and phpinfo(INFO_ENVIRONMENT) are three completely distinct environment stores. doing putenv("x=y") does not affect $_ENV; but also doing $_ENV["x"]="y" likewise does not affect getenv("x"). And neither affect what is returned in phpinfo().

Assuming the USER environment variable is defined as "dave" before running the following:

<?php
print "env is: ".$_ENV["USER"]."\n";
print "(doing: putenv fred)\n";
putenv("USER=fred");
print "env is: ".$_ENV["USER"]."\n";
print "getenv is: ".getenv("USER")."\n";
print "(doing: set _env barney)\n";
$_ENV["USER"]="barney";
print "getenv is: ".getenv("USER")."\n";
print "env is: ".$_ENV["USER"]."\n";
phpinfo(INFO_ENVIRONMENT);
?>

prints:

env is: dave
(doing: putenv fred)
env is: dave
getenv is: fred
(doing: set _env barney)
getenv is: fred
env is: barney
phpinfo()

Environment

Variable => Value
...
USER => dave
...

The other problem with the code from av01 at bugfix dot cc is that
the behaviour is as per the comments here, not there:
<?php
putenv('MYVAR='); // set MYVAR to an empty value.  It is in the environment
putenv('MYVAR'); // unset MYVAR.  It is removed from the environment
?>

Values of variables with dots in their names are not output when using getenv(), but are still present and can be explicitly queried.

(saw this behaviour using PHP 8.2.4)

<?php

// set
putenv('foo.bar=baz');

// dump all
var_dump(getenv());    # <== variable 'foo.bar' NOT included, its value is not dumped

// dump explicitely 'foo.bar'
var_dump(getenv('foo.bar'));    # works, value 'baz' is shown

White spaces are allowed in environment variable names so :

<?php
putenv('U =33');
?>

Is not equivalent to 

<?php
putenv('U=33');
?>

It's the putenv() type of environment variables that get passed to a child process executed via exec().

If you need to delete an existing environment variable so the child process does not see it, use:

putenv('FOOBAR');

That is, leave out both the "=" and a value.

Environment variables are part of the underlying operating system's
way of doing things, and are used to pass information between a parent
process and its child, as well as to affect the way some internal
functions behave.  They should not be regarded as ordinary PHP
variables.

A primary purpose of setting environment variables in a PHP script is
so that they are available to processes invoked by that script using
e.g. the system() function, and it's unlikely that they would need to
be changed for other reasons.

For example, if a particular system command required a special value
of the environment variable LD_LIBRARY_PATH to execute successfully,
then the following code might be used on a *NIX system:

<?php
 $saved = getenv("LD_LIBRARY_PATH");        // save old value
 $newld = "/extra/library/dir:/another/path/to/lib";  // extra paths to add
 if ($saved) { $newld .= ":$saved"; }           // append old paths if any
 putenv("LD_LIBRARY_PATH=$newld");        // set new value
 system("mycommand -with args");        // do system command; 
                        // mycommand is loaded using
                        // libs in the new path list
 putenv("LD_LIBRARY_PATH=$saved");        // restore old value
?>

It will usually be appropriate to restore the old value after use;
LD_LIBRARY_PATH is a particularly good example of a variable which it
is important to restore immediately, as it is used by internal
functions.

If php.ini configuration allows, the values of environment variables
are made available as PHP global variables on entry to a script, but
these global variables are merely copies and do not track the actual
environment variables once the script is entered.  Changing
$REMOTE_ADDR (or even $HTTP_ENV_VARS["REMOTE_ADDR"]) should not be
expected to affect the actual environment variable; this is why
putenv() is needed.

Finally, do not rely on environment variables maintaining the same
value from one script invocation to the next, especially if you have
used putenv().  The result depends on many factors, such as CGI vs
apache module, and the exact way in which the environment is
manipulated before entering the script.