Hell-Volhard-Zelinski Reaction
TL;DR: When $\ce{PBr3/PCl3}$ is not available as reagent, red phosphorus is used which is reacted to bromine or chlorine to form the reagents in situ.
Long answer:
Hell–Volhard–Zelinsky halogenation reaction halogenates carboxylic acids at the α-carbon.  The reaction is initiated by addition of a catalytic amount of $\ce{PBr3}$, after which one molar equivalent of $\ce{Br2}$ is added. $\ce{PBr3}$ replaces the carboxylic -OH with a bromide, resulting in a carboxylic acid bromide. The acyl bromide can then tautomerize to an enol, which will readily react with the $\ce{Br2}$ to brominate a second time at the α position.


Now, what happens if $\ce{PBr3}$ is not available?

You can use red phosphorus as a reagent. It is reasonably air stable (whereas white phosphorus combust in air almost spontaneously). Phosphorus reacts with bromine to give phosphorus tribromide which helps in the conversion of carboxylic acid into an acyl bromide.
$$\ce{2P_{(red)} + 3Br2 -&gt; 2PBr3}$$

Why exactly $\ce{PBr3/PCl3}$ is needed and why an extra molar equivalent of
$\ce{Br2/Cl2}$ is needed?

When left out in the air, $\ce{PBr3}$ will readily combine with any water vapor present to form a new $\ce{P-O}$ bond along with $\ce{HBr}$. The strength of the $\ce{P-O}$ bond [about $\pu{140 kcal/mol}$] makes $\ce{PBr3}$ very useful for swapping out $\ce{C-O}$ bonds for $\ce{C-Br}$ bond and that's how carboxylic acids gets converted to acyl bromides.
However, $\ce{PBr3}$ by itself is not sufficient to perform the HVZ reaction. A molar excess of $\ce{Br2}$ is added to the already present $\ce{PBr3}$, which perform the halogenation of the alpha-carbon. That's why the reagent is always mentioned as ($\ce{PBr3 + Br2}$).
References

https://en.wikipedia.org/wiki/Hell%E2%80%93Volhard%E2%80%93Zelinsky_halogenation
https://www.organic-chemistry.org/namedreactions/hell-volhard-zelinsky-reaction.shtm
https://www.masterorganicchemistry.com/2020/08/26/the-hellvolhardzelinsky-reaction/


That generates another question: Can we use PI3 as reagent in HVZ reaction?