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A do-while loop is a post-test control flow structure that guarantees the execution of its code block at least once. Unlike a standard while loop, which evaluates its condition prior to execution, the do-while loop evaluates the boolean condition at the end of the iteration.

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Syntax

do {
    // Statements to execute
} while (condition);
If the loop body consists of only a single statement, the curly braces {} are optional. Variables must be initialized prior to use to avoid undefined variable warnings in modern PHP: 
$i = 0;
do $i++; while ($i < 5);

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Execution Flow

  1. Execution: The PHP interpreter enters the do block and executes the enclosed statements sequentially.
  2. Evaluation: Upon reaching the while clause, the condition expression is evaluated in a boolean context.
  3. Iteration or Termination:
    • If the condition evaluates to true, the execution pointer jumps back to the beginning of the do block.
    • If the condition evaluates to false, the loop terminates, and control passes to the next sequential statement in the script.

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Technical Characteristics

  • Trailing Semicolon: The while (condition) statement at the end of the loop must be explicitly terminated with a semicolon (;). Omitting this will result in a parse error.
  • Guaranteed Initial Execution: Because the condition is evaluated at the bottom of the loop, the code block will always execute the first time, even if the condition is inherently false at the start of the execution cycle.
  • Scope: PHP lacks block-level scope for variables. Variables declared or initialized inside the do block are fully accessible within the while condition and in the surrounding global or function scope following the loop.
  • Syntax Constraints: Unlike the standard while loop, which supports an alternative syntax for templating (while (...): ... endwhile;), the do-while loop does not support an alternative syntax.
  • Control Statements:
    • break: Immediately terminates the loop and transfers control to the statement following the loop.
    • continue: Skips the remaining statements in the current iteration and immediately jumps to the while condition evaluation to determine if the next iteration should begin.

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Code Example

The following example demonstrates the post-test nature of the loop. Even though the condition evaluates to false immediately, the execution block runs exactly once. 
<?php
$iterator = 10;

do {
    echo "Current value: " . $iterator . "\n";
    $iterator++;
} while ($iterator < 5);

// Output: Current value: 10
// The loop terminates after the first iteration because 11 is not less than 5.
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