What does this SQL statement do? - php

Question

I am trying to understand this SQL statements :
$id = 5;
$stmt = $conn->prepare('SELECT * FROM myTable WHERE id = :id');
$stmt->execute(array('id' => $id));
while($row = $stmt->fetch()) {
print_r($row);
}
Can someone please explain me step by step what exactly is going on here?
From what i understand :
$stmt = $conn->prepare('SELECT * FROM myTable WHERE id = :id');
1) $stmt is about to take as iinput an SQL query. The SQL query is to select all the rows from a table that their id is equal to 5.
$stmt->execute(array('id' => $id));
2) We execute the statement. Now the $stmt has these rows?
$row = $stmt->fetch()
3) This is the most confusing line for me. What exactly happens here? Variable "row" takes one by one the rows that have id = 5 ? Is that what fetch() does ? And if yes , how exaxtly does it return the results? Its an array of all the correct answers? EG all the rows that have id = 5 ? I dont understand how exactly this while loop works here.The first time it runs "row" will have the first row ? The second time it runs , will have the second row that satisfies our creteria (id = 5) and so on? Is it like that every time i run fetch one result will be returned? And next time i run fetch , the next result , till there is no more result to satisfy the query?
I thing i am so close to get this one. Anything that could help me understand it completely would be highly appreciated !

Answer 1

I'll explain as comments:
$id = 5;
// Create a prepared statement - don't actually execute the statement yet.
// The :id value in the statement will be replaced by a parameter value (safely) when the
// statement is executed
$stmt = $conn->prepare('SELECT * FROM myTable WHERE id = :id');
// Execute the statement against the DB - the $stmt var now contains the result set for the
// executed statement. e.g. it contains *all* the results that the query fetched
$stmt->execute(array('id' => $id));
// Now we loop through the rows in the result set (they are all in memory at this point).
// "fetch" will start from row 1 and return the next result each time you call it again.
// when there are no more rows it returns FALSE and therefore breaks out of the while loop
while($row = $stmt->fetch()) {
print_r($row);
}
Just checking docs also and whilst this is how it was done previously (been years since I've touched PHP) it looks like stmt->fetch() actually places results into bound variables:
http://php.net/manual/en/mysqli-stmt.fetch.php
$row = array();
stmt_bind_assoc($stmt, $row);
// loop through all result rows
while ($stmt->fetch())
{
print_r($row);
}
Does the code you originally posted actually work? It doesn't appear you bind any variables and therefore since the $stmt-fetch() call returns bool TRUE/FALSE it would seem to be that $row would not get set to anything but TRUE/FALSE

Answer 2

here it uses PDO for execution,
Repeated SELECT using prepared statements through which you can call repeated query
$stmt = $conn->prepare('SELECT * FROM myTable WHERE id = :id');
it defines the prepared statement where :id is placeholder
$stmt->execute(array('id' => $id));
this places assigns the value to placeholder and execute the query
$row = $stmt->fetch()
it fetch the record from select
for more reference visit the link
http://www.php.net/manual/en/pdo.prepared-statements.php