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The if statement is a fundamental control flow structure in PHP that dictates the conditional execution of a code block based on the evaluation of a specified expression. If the expression evaluates to true, the nested statement or code block is executed; if it evaluates to false, the execution bypasses the block.

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Standard Syntax

PHP utilizes C-style syntax for standard control structures, enclosing the conditional expression in parentheses and the executable block in curly braces. 
if (expression) {
    // statement(s) to execute if expression evaluates to true
}
If only a single statement is to be executed, the curly braces can technically be omitted. However, modern PHP coding standards (such as PER/PSR-12) mandate the use of curly braces for all control structures to prevent scope-related bugs during maintenance. 
// Valid, but discouraged by coding standards
if (expression)
    statement;

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Expression Evaluation and Type Juggling

The expression inside the if statement does not need to be a strict boolean type. PHP utilizes implicit type casting (type juggling) to evaluate the expression in a boolean context. The following values are implicitly cast to false (often referred to as “falsy” values):
  • The boolean false itself
  • The integer 0 (zero)
  • The float 0.0 (zero)
  • The empty string "", and the string "0"
  • An array with zero elements []
  • The special type NULL (including unset variables)
  • SimpleXML objects created from empty tags
Any value not strictly matching the list above evaluates to true.

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Branching: else and elseif

The if statement can be extended using else to define a fallback block when the initial expression evaluates to false, and elseif to evaluate subsequent conditions. 
if (expression1) {
    // executed if expression1 is true
} elseif (expression2) {
    // executed if expression1 is false and expression2 is true
} else {
    // executed if all preceding expressions are false
}
Note: PHP accepts both elseif (single word) and else if (two words) when using curly braces. The parser treats them identically in this context.

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Alternative Syntax

PHP provides an alternative syntax for control structures, which is primarily utilized within view templates mixed with HTML. This syntax replaces the opening curly brace with a colon (:) and the closing curly brace with the endif; keyword. 
if (expression1):
    // statement(s)
elseif (expression2):
    // statement(s)
else:
    // statement(s)
endif;
Crucial Distinction: When using the alternative syntax, you must use the single-word elseif. Using the two-word else if with the colon syntax will result in a parse error.
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