How can I make a specific PHP function

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PHP

Hello guys, I am newbie in PHP. I have a problem I want to make a php function that will check the number if it is less than 75. I made a PHP function but I am not sure if this is correct. This is my old code an I want to simplify it.
if($_POST[grade] < 75)
{
echo 'Invalid grade';
}
elseif($_POST['grade2'] < 75)
{
echo'invalid grade';
}
Here is my current php function code:
function numeric($string)
{
return ....... i dont know the next
}
thanks in advance guys