If compatibility with various PHP versions is especially important to your script, it's useful to be able to check for the existence of functions. The function function_exists does that what you'd expect. It takes a string with a function's name and returns TRUE or FALSE depending on whether the function has been defined.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="EN" lang="EN">
<head>
<title>PHP</title>
<meta Name="Author" Content="Hann So">
</head>
<body>
<p>
<?php
$test=function_exists("test_this");
if ($test == TRUE)
{
echo "Fucntion test_this exists.";
}
else
{
echo "Function test_this does not exist.";
}
?>
</p>
</body>
</html>
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