In PHP, the if, else, and elseif statements are used for conditional execution of code. These statements allow you to control the flow of your program based on whether a certain condition is true or false.

1.  if Statement:

The if statement executes a block of code if a specified condition is true.

<?php
$number = 10;

if ($number > 0) {
    echo "The number is positive.";
}
?>

2.if...else Statement:

The if...else statement executes one block of code if a specified condition is true and another block if it's false.

<?php
$number = -5;

if ($number > 0) {
    echo "The number is positive.";
} else {
    echo "The number is non-positive.";
}
?>

3. if...elseif...else Statement:

The if...elseif...else statement allows you to specify multiple conditions and execute different blocks of code based on the first true condition.

<?php
$number = 0;

if ($number > 0) {
    echo "The number is positive.";
} elseif ($number < 0) {
    echo "The number is negative.";
} else {
    echo "The number is zero.";
}
?>

You can use these conditional statements to control the flow of your program based on various conditions. Make sure to properly structure your code with indentation and braces to maintain readability. Additionally, you can nest if statements within other if statements for more complex conditions.