Convert a MYSQL* query into a PDO ready statement with placeholders

Question

Im having real trouble converting a SQL query from mysql to a PDO prepared statement. It is the following and it is used to calculate the distance between a coordinate stored in a session (the users position) and a coordinate in the database, and if it is within a radius determined by the user:

$query = "
SELECT 
* 
FROM 
first_page_data 
WHERE 
((((acos(sin((".$_SESSION['alat']."*pi()/180)) * 
sin(('geo_lat1'*pi()/180))+cos((".$_SESSION['alat']."*pi()/180)) * 
cos(('geo_lat1'*pi()/180)) * cos(((".$_SESSION['alon']."- 'geo_lon1') * 
pi()/180))))*180/pi())*60*1.1515) * (1.609344/1000)) < ".$_SESSION['aradius'];

I want to change the session variables, and the geo_lat and geo_lon variables into placeholders so I can bind the values to them but I cant for the life of me get it to work!

I dont even know how to see how im getting on because once ive replaced the variables above with placeholders (ive been using the unnamed questionmark placeholders) and then binded the values to them, I dont know how to retrieve the compiled query from $stmt before I execute it:

$stmt = db->prepare($query);
$stmt->bindValue(1, $_SESSION['alat'], PDO::PARAM_STR);
**more bindings**
$stmt->execute();   

Answer 1

These changes are very trivial. I put in placeholder names, removed the string concatenation, and removed the incorrect quotes. It could use help with formatting, or perhaps a UDF.

# Change string-concat for placeholders
$query = "
SELECT 
* 
FROM 
first_page_data 
WHERE 
((((acos(sin(( :lat1 *pi()/180)) * 
sin((geo_lat1*pi()/180))+cos(( :lat2 *pi()/180)) * 
cos((geo_lat1*pi()/180)) * cos((( :lon - geo_lon1) * 
pi()/180))))*180/pi())*60*1.1515) * (1.609344/1000)) < :rad";

# Bind values (PARAM_STR is default)
# While PDO does support using a named parameter multiple times
# when emulating placeholders, this is not guaranteed. To be safe,
# here the code is binding the same value to two "different" params.
$stmt = db->prepare($query);    
$stmt->bindValue(":lat1", $_SESSION['alat']);
$stmt->bindValue(":lat2", $_SESSION['alat']);
$stmt->bindValue(":lon", $_SESSION['along']);
$stmt->bindValue(":rad", $_SESSION['aradius']);

# Execute! I recommend enabling PDO Exceptions to avoid so much
# manual error-checking of result values.
$stmt->execute();   

Now, there is no direct way to see the statement with data as that's not how placeholders work - even if PDO internally emulates placeholders it's merely an implementation detail and you don't get to see it.

However, there are various solutions to the more general task of "debugging statements" discussed in How to debug PDO database queries? (I recommend using the database logging for development as your code probably shouldn't be logging or printing queries directly.)